Six healthy adult male volunteers underwent serial blood drawings at 4-hour intervals over 24 hours for the definition of melatonin (MT), prolactin (PRL), cortisol, and testosterone circadian patterns. Serum levels of triiodotironine (T3) and thyroxine (T4) were determined at 0800. Systolic and diastolic blood pressure and heart rate were automatically recorded every 30 minutes for 24 hours. The responses of luteinizing hormone (LH), follicle stimulating hormone (FSH), PRL, thyroid stimulating hormone (TSH), cortisol, and aldosterone to a stimulation test with gonadotrophin-releasing hormone (Gn-RH), thyrotrophin-releasing hormone (TRH), adrenocorticotrophin (ACTH), and testosterone to human chorionic gonadotrophin (HCG) were also evaluated. The same protocol was repeated after a two-month course of treatment with MT, 2 mg per os daily at 1800. After treatment, we recorded a marked elevation of mean serum MT levels with a significant phase-advance of its circadian rhythm. The 24-hour patterns of cortisol and testosterone displayed an anticipation of the morning acrophase of about 1.5 hour (not significant) for cortisol and three hours (P less than 0.05) for testosterone. PRL pattern was unchanged as well as serum levels of thyroid hormones. The circadian organization of the cardiovascular variables did not show any changes after MT supplementation; the pituitary, adrenal, and testicular responses to specific stimuli were comparable before and after treatment. These results are compatible with the view that the MT signal may provide temporal cues to the neuroendocrine network for the organization of testicular circadian periodicity.