The pupillary effects of the partial opiate agonist buprenorphine were studied in 16 male subjects who were heroin dependent at the time of admission to the study. Sublingual buprenorphine (8 mg) was administered daily for 18 days and continued either daily or on alternate days from study days 19 through 36. On days 37 through 56, all subjects received buprenorphine placebo. Compared to placebo, buprenorphine decreased pupil size and diminished the constriction and dilation velocities of the light reflex. These effects occurred within 5 hours of buprenorphine administration. Following placebo administration during alternate-day dosing of buprenorphine, pupil size increased and constriction and dilation velocities of the light reflex were significantly greater than after buprenorphine administration in the same subjects. This pattern of effects was observed after buprenorphine was discontinued (day 37). The results indicated that buprenorphine has pupillary effects like those of full opiate agonists. The time course of these effects was similar to previously-reported effects of buprenorphine on the electroencephalogram but not to the time course of subjective effects.