The development of cross-tolerance between methadone and heroin was studied in postaddict volunteers who had been drug-free for at least 6 weeks. Two methadone dose schedules were used; each was employed in six subjects. One schedule brought the subjects to a dose of 40 mg, while the other brought them to 80 mg of methadone a day. Subjects received injections of heroin (0.214 mg/kg) and placebo at various times before and during methadone treatment. Pupillary and subjective effects of injections were measured. Plasma levels of methadone were determined concurrently. Subjects on both treatment schedules developed an incomplete cross-tolerance to this dose of heroin. As the dose and plasma level of methadone increased with time, the cross-tolerance to all heroin effects increased. Plasma levels did not affect the development of cross-tolerance independently of methadone dose. The most important contribution to the cross-tolerance to pupillary effects was made by the duration of methadone treatment. Furthermore, the cross-tolerance to the subjective effects of heroin developed earlier than that to the pupil effect.