Psychomotor performance and memory were assessed in 6 patients with seasonal affective disorder following one week of daily administration of oral melatonin. No effect was noted on either vigilance or memory testing, but reaction time on a simple visual/tactile task was significantly reduced. Identical testing following a week of placebo administration showed no changes from baseline. Clinical status was monitored by both observer and self ratings; no clinical changes were detected that could account for the performance difference. The reaction time finding is in contrast to a previous report of a reaction time increase following melatonin administration. We discuss the possibility that this discrepancy is due to different dosages of melatonin administered or to the effects of melatonin on circadian rhythms of performance.