In these habeas corpus proceedings, the petitioner, a federal prisoner, alleges that the U.S. Parole Commission fixed his presumptive parole release date using guidelines that violated the ex post facto clause of article 1, section 9, of the Constitution. The district court denied relief. We affirm.
I.
The petitioner, Douglas Dufresne, was a senior pilot for Pan American Airlines. During the late 1970’s he began trafficking in illicit drugs, at one point importing eight pounds of raw opium into the United States from Iran. In 1979 a federal grand jury in the Western District of Texas indicted petitioner in three counts for conspiracy to import opium, possession with intent to distribute opium, and conspiracy to import cocaine. The indictment alleged that petitioner committed these acts during the period from March 1978 to March 1979.
On September 17, 1979, petitioner pled guilty to the third count of the indictment,1 conspiracy to import cocaine, and the district court sentenced him to a partially indeterminate eight-year prison sentence under 18 U.S.C. § 4205(a) (1982).2 Under this sentence, petitioner would become eligible for parole at the discretion of the U.S. Parole Commission (the Commission) after serving one-third of his eight-year term. Id. On January 14, 1980, the court granted petitioner’s Fed.R.Crim.P. 35 motion to modify sentence and made petitioner’s sentence fully indeterminate under 18 U.S.C. § 4205(b)(2) (1982).3 Petitioner thus became eligible for parole at any time, at the Commission’s discretion.
*1545The Commission, acting through the Commissioner in charge of the Southeast Region (regional Commissioner),4 initially considered petitioner for parole four months later, in May. Following the recommendation of the two examiners who conducted petitioner’s parole hearing and applying the Commission’s parole guidelines, 28 C.F.R. § 2.20 (1979), the regional Commissioner gave petitioner a presumptive parole release date of fifty-two months. This meant that petitioner would be paroled in fifty-two months if he “substantially observed the rules of the institution” in which he was confined, see 18 U.S.C. § 4206(a) (1982); if he did not, the Commission could require him to serve more of his eight-year sentence.
In setting petitioner’s presumptive release date, the regional Commissioner applied guidelines which focused upon the severity of the petitioner’s offense and his parole prognosis. These parole guidelines classified the severity of his offense as “Greatest I,” a severity rating surpassed only by the “Greatest II” rating given to crimes such as murder and air piracy. The severity rating was high because at the time petitioner committed his crime he possessed a managerial interest in more than fifty grams of 100% pure opiate.5 The guidelines gave petitioner a “very good” parole prognosis because his perfect eleven “salient factor score,” calculated by weighing offender characteristics such as a history of criminal convictions, previous incarceration, and drug dependency, indicated that he would pose no risk to society if released. According to the guidelines, an inmate with an offense severity rating of “Greatest I” and a salient factor score of eleven was to serve a term somewhere between forty to fifty-two months. The regional Commissioner set petitioner’s term at fifty-two months.6
Petitioner appealed this decision to the Commission’s National Appeals Board. He contended that the regional Commissioner’s decision should have been based on the offense severity rating in effect at the time he committed his crime rather than the one currently in effect. The earlier offense rating, repealed in mid-1979,7 would have assessed the seriousness of petitioner’s crime on the basis of the street value of the drugs he possessed, not on the basis of the purity of those drugs and his managerial interest therein. Thus, because his drugs had a street value of less than $100,000, petitioner’s crime would have been classified as “Very High,” not “Greatest I.” A “Very High” offense rating, combined with a salient factor score of eleven, would have called for petitioner to be incarcerated for a period between twenty-six to thirty-six months.
The National Appeals Board affirmed the regional Commissioner’s decision, and petitioner, having exhausted his administrative remedies, brought these habeas corpus proceedings in the district court. Petitioner sought an order requiring the Commission to give him a presumptive parole date within the time frame specified in the guidelines in effect when he committed his *1546crime. Petitioner’s sole complaint was that the Commission, in applying the current guidelines, retroactively enhanced the punishment for his crime, thereby violating the ex post facto prohibition found in article 1, section 9, clause 3, of the Constitution. The parties consented to the trial of the case before a U.S. Magistrate. See 28 U.S.C. § 636(c) (1982). The magistrate agreed with petitioner and granted the relief he sought. The district court, on appeal, id,., reversed, concluding that the parole guidelines the Commission applied in petitioner’s case neither deprived him of any pre-existing right nor enhanced his sentence; rather, they merely aided the Commission in the exercise of its decision making. Petitioner took this appeal from the district court’s decision.
II.
The ex post facto clause at issue commands that “[n]o ... ex post facto law shall be passed.” U.S. Const. Art. I, § 9, cl. 3.8 An ex post facto law possesses three characteristics: it is a criminal9 or penal measure, retrospective, and disadvantageous to the offender because it may impose greater punishment. Weaver v. Graham, 450 U.S. 24, 28-29, 101 S.Ct. 960, 964, 67 L.Ed.2d 17 (1981); Dobbert v. Florida, 432 U.S. 282, 292-93, 97 S.Ct. 2290, 2297-98, 53 L.Ed.2d 344 (1977). See Paschal v. Wainwright, 738 F.2d 1173, 1175 (11th Cir.1984). A law which is merely procedural and does not add to the quantum of punishment, however, cannot violate the ex post facto clause even if it is applied retrospectively. Weaver, 450 U.S. 24, 29 n. 12, 101 S.Ct. 960, 964 n. 12, 67 L.Ed.2d 17 (1981); see Dobbert, 432 U.S. 282, 293, 97 S.Ct. 2290, 2298, 53 L.Ed.2d 344 (1977) (“[e]ven though it may work to the disadvantage of a defendant, a procedural change is not ex post facto.”). See also Thompson v. Missouri, 171 U.S. 380, 18 S.Ct. 922, 43 L.Ed. 204 (1898); Hopt v. Utah, 110 U.S. 574, 4 S.Ct. 202, 28 L.Ed. 262 (1884).
When subjecting a law to ex post facto scrutiny, courts should bear in mind the related aims of the ex post facto clause: providing fair notice of which acts will subject the perpetrator to criminal sanctions,10 and preventing vindictive criminal legislation.11 We are guided by these principles *1547as we determine whether petitioner has presented an ex post facto violation.
The second characteristic of an ex post facto law is concededly present in this case; the guidelines applied retrospectively to petitioner. The first and third characteristics are not present, however; the guidelines neither worked to petitioner’s detriment nor constituted a criminal or penal law.
The federal sentencing statute under which petitioner received his modified sentence, 18 U.S.C. § 4205(b)(2) (1982), was enacted as a “medical model” sentencing provision.12 The sentencer was to utilize the fully indeterminate prison term, prescribed by the statute, primarily for the purpose of rehabilitating the defendant; the parole commission, as “doctor,” would monitor the defendant’s response to treatment, decide when he was cured, and then release him.13
In 1976, the Congress, having grown dissatisfied with this medical model sentencing scheme for several reasons, passed the Parole Commission and Reorganization Act (the Parole Act or Act).14 The Act reconstituted the parole authority as the U.S. Parole Commission, abolished the parole authority’s “doctor” role, and gave it the new role of resentencer. Henceforth, the Commission, acting on essentially the same information presented to the sentencing judge,15 would resentence the defendant for the purposes of punishment, general deterrence and specific deterrence. Rehabilitation would not be one of the Commission’s resentencing objectives; any rehabilitation an offender might obtain would be a coincidental result of his incarceration.
The Parole Act instructed the Commission to carry out a “national parole policy” and to promulgate guidelines to aid it in making parole decisions. 18 U.S.C. § 4203(a)(1) (1982). Following this directive, the Commission promulgated guidelines for the announced purpose of promoting “a more consistent exercise of discretion, and enabling fairer and more equitable decision-making without removing individual case consideration.” 28 C.F.R. § 2.20 (1979).
The Act requires the Commission, in deciding whether to grant parole, to consider “the nature and circumstances of the [prisoner’s] offense and the history and characteristics of the prisoner” and its guidelines. 18 U.S.C. § 4206(a) (1982). The Commission’s guidelines speak to “the nature and circumstances of the [prisoner’s] offense” *1548by classifying it in terms of severity, and to “the history and characteristics of the prisoner” by looking to his prior criminal record, drug dependence and other “salient factors” that are presumed to indicate whether he will be a good parole risk. The Commission is bound to follow these guidelines unless there is “good cause” for not doing so. 18 U.S.C. § 4206(c) (1982).16
The Act cautions the Commission not to parole a prisoner if he has not “substantially observed the rules of the institution or institutions to which he has been confined,” if his “release would ... depreciate the seriousness of his offense or promote disrespect for the law,” or if his “release would ... jeopardize the public welfare.” 18 U.S.C. § 4206(a) (1982). With this sentencing parole scheme in mind, we turn to the two ex post facto issues we must resolve: first, whether the Commission, in amending its guidelines to upgrade the severity rating of petitioner’s offense from “Very High” to “Greatest II,” increased his punishment; second, whether the guidelines are “laws.”
A.
The ex post facto clause operates to ensure that a citizen’s fair notice as to what acts are criminal will not be changed after the citizen acts in reliance on that notice. When petitioner conspired to import cocaine into the United States he was on notice of the prison sentence he could receive if convicted, the portion of that sentence he would have to serve before he would be eligible for parole, and the extent to which his period of incarceration could be reduced through gain time. See, e.g., Weaver v. Graham, 450 U.S. 24, 29, 101 S.Ct. 960, 964, 67 L.Ed.2d 17 (1981). Specifically, petitioner was on notice that, if he received the eight-year fully indeterminate sentence the court imposed, the Commission would have the discretion to parole him at any time, unless it determined at the time it considered him for parole that his release would “depreciate the seriousness of his offense or promote disrespect for the law” or would “jeopardize public welfare.” Petitioner was also on notice that, in considering him for release, the Commission had to follow its guidelines unless “good cause” indicated otherwise.
Petitioner’s claim that the Commission could not amend the guidelines retrospectively if the amendment would produce a longer term of incarceration implies that he was not on notice, when he committed his crime, that such an amendment could occur. In truth, petitioner was on notice that such an amendment might well occur. The Commission had a statutory duty to monitor and periodically update its guidelines and to apply current guidelines to crimes previously committed.17 See, e.g., H.Conf.Rep. No. 838, 94th Cong.2d Sess. 25, reprinted in 1976 U.S.Code Cong. & *1549Ad.News, 335, 358-59 (in determining release Commission “shall be cognizant of the public perception of and respect for the law.”) In the situation before us, the Commission determined that the street value of the narcotics an offender possessed did not reflect upon the seriousness of his drug offense as much as did the purity of the narcotics and his managerial interest in them. When he committed his crime, petitioner knew that the Commission, prior to his parole hearing, might make such a judgment call and thus lengthen any period of confinement he might have to serve. Petitioner knew, or should have known, that the role he played in his smuggling operation and the purity of the drugs involved were highly relevant to two important questions the Commission would have to decide in considering him for parole: whether his release would (1) depreciate the seriousness of his offense or promote disrespect for law and (2) jeopardize the public welfare.
Six U.S. Courts of Appeals and one U.S. Supreme Court Justice have entertained claims similar to petitioner’s and have held that retrospective changes in parole guidelines do not aggravate the punishment provided for a crime. See Portley v. Grossman, 444 U.S. 1311, 100 S.Ct. 714, 62 L.Ed.2d 723 (1980) (Rehnquist, J., circuit justice; motion for stay); Warren v. United States Parole Commission, 659 F.2d 183 (D.C.Cir.1981), cert. denied, 455 U.S. 950, 102 S.Ct. 1454, 71 L.Ed.2d 665 (1982); Hayward v. United States Parole Commission, 659 F.2d 857 (8th Cir.1981), cert. denied, 456 U.S. 935, 102 S.Ct. 1991, 72 L.Ed.2d 454 (1982); Rifai v. United States Parole Commission, 586 F.2d 695 (9th Cir.1978); Shepard v. Taylor, 556 F.2d 648 (2d Cir.1977); Ruip v. United States, 555 F.2d 1331 (6th Cir.1977).18 In Portley, a reparole case, Justice Rehnquist sitting as a circuit justice, held that changes in parole guidelines do not add punishment retrospectively. He noted that the ex post facto clause does not extend to every change which may work to a defendant’s disadvantage and does not limit changes in procedure which do not affect substantive rights. 444 U.S. at 1312, 100 S.Ct. at 715 (citing Dobbert, 432 U.S. at 293, 97 S.Ct. at 2298). Because of the discretion retained by the Commission, Justice Rehnquist held that the guideline amendments were merely procedural, no pre-existing right was impaired, and the offender’s punishment was not enhanced. Id. at 1312-13, 100 S.Ct. at 714-15.19
In sum, the Commission’s decision in this case implicated neither of the principal policies served by the ex post facto clause, to ensure that the offender is not singled out as the target of legislative retribution and that he is provided fair notice of the punishment that may be meted out if he transgresses the criminal law.
B.
As the district court concluded, the Commission’s guidelines are, simply, guidelines. They are not promulgated by any *1550legislature. Rather, they are stated policy rules that show how agency discretion is likely to be exercised.20 They do not state rules of conduct for the public. A change in the guidelines does not affect the maximum or minimum prison sentence a court may impose, the point at which the prisoner becomes eligible for parole or his mandatory release date on good time.21 Accord, Portley v. Grossman, 444 U.S. 1311, 1312-13, 100 S.Ct. 714, 715, 62 L.Ed.2d 723 (1980). The guidelines merely indicate when, in most cases, the prisoner can expect release. We agree with the second, sixth, seventh, and ninth circuits that the parole guidelines do not have the force and effect of law. Zeidman v. United States Parole Commission, 593 F.2d 806 (7th Cir.1979); Rifai v. United States Parole Commission, 586 F.2d 695 (9th Cir.1978); Shepard v. Taylor, 556 F.2d 648 (2d Cir.1977); Ruip v. United States, 555 F.2d 1331 (6th Cir.1977); accord, Joost v. United States Parole Commission, 535 F.Supp. 71 (D.Kan.1982); Kirby v. United States, 463 F.Supp. 703 (D.Minn.), vacated on other grounds, 600 F.2d 146 (8th Cir.1979); Wilson v. United States Parole Commission, 460 F.Supp. 73 (D.Minn.1978). As the Sixth Circuit stated:
[W]hat is involved in this case is not agency interpretation of law but an agency’s setting up guidelines for itself to assure the uniform execution of its business. These guidelines are not law, but guideposts which assist the Parole Commission ... in exercising its discretion. Nor do these guidelines have the characteristics of law. They are not filed and rigid, but are flexible. The Commission remains free to make parole decisions outside of these guidelines.
Ruip, 555 F.2d at 1335.
Even a showing that the Commission adhered to the guidelines in the vast majority of cases22 would not necessarily prove that the guidelines were law; otherwise, every policy that an agency adhered to consistently would become a law from which the agency could not vary. Accord, Rifai, 586 F.2d at 698. Congress intended that the Commission periodically amend the guidelines to reflect contemporary views concerning the seriousness of given crimes and parole recidivism. E.g., S.Rep. No. 369, 94th Cong., 2d Sess. 18, reprinted in 1976 U.S.Code Cong. & Ad. News 335. Such amendments are not indicative of provisions which have the force and effect of law. As the legislative history shows, “[t]he standards for release on parole [in the Parole Act] ... are not changed from [prior] law.” Id. at 339. The Commission may follow its guidelines, disregard them, or change them. Parole remains an act of discretion. See, e.g., Shahid v. Crawford, 599 F.2d 666, 669 (5th Cir.1979); Brown v. Lundgren, 528 F.2d 1050, 1055 (5th Cir.), cert. denied, 429 U.S. 917, 97 S.Ct. 308, 50 L.Ed.2d 283 (1976).
Because the guidelines in question are not criminal laws and their amendment did not add to the punishment prescribed for petitioner’s crime, petitioner has failed to establish two of the requisite elements of an ex post facto claim. Accordingly, the judgment of the district court denying his claim is
AFFIRMED.