The circadian rhythm of plasma melatonin was investigated in normal men 18-30 years (N = 5), normal men 50-70 years (N = 5) and in six patients with endogenous depression. The environmental photoperiod was 11 hours. The subjects and patients were indoors with lights on from 07:00 until 23:00 hours. Blood samples were obtained every 4 hours over a 24 hour period, with additional sampling at 22:00 and 02:00 hours. Plasma melatonin was estimated by radioimmunoassay compared to both groups of controls. In the depressed patients, the levels of melatonin were low throughout the 24 hour period. The depressives had a delayed onset of the dark phase of the rhythm. The patients also showed peak melatonin levels occurring earlier than in the controls. Circadian rhythm of melatonin and therefore of its pacemaker may be altered in endogenous depression.