Differences and similarities in the temporal organization of hormone secretion in plasma reflect the activity of CNS pacemakers. One aspect of this activity, the temporal synchronization of the secretion of different hormones is still poorly understood. We report the analysis of melatonin and testosterone plasma concentrations during two nights in 6 normal healthy young men. Blood was collected every 20 min between 2040 and 0640. Plasma testosterone concentrations increased by 1.5- to 2- fold during the second part of the night, and melatonin by 2.5-to 4-fold. In each subject. the individual temporal pattern of melatonin was quite stable over the two nights of sampling. while testosterone profiles showed fluctuations. There was a high degree of parallelism in these two hormones nocturnal secretion. These results, together with previous studies, suggest that melatonin might entrain the nocturnal secretion of testosterone