The effect of melatonin on LH and FSH secretion in basal conditions and after stimulation with synthetic LHRH was studied in 3 volunteer normal men and 3 women two to five years after menopause. During the first three days of study, blood samples were obtained at 8 AM. On the second day, an iv injection of 50 mug LHRH was performed; on the third day, 2 ml of 0.9% saline were injected. In both cases, blood samples were obtained 30 and 60 minutes after the injection. On the fourth day, the subjects began melatonin (10 mg daily im for 13 days). Blood samples at 8 AM were obtained after 5, 11, and 13 days of administration of the drug. On days 5 and 11 of the treatment, iv injections of 50 mug LHRH were performed and on day 13 an iv injection of saline 0.9% was given. Blood samples were obtained 30 and 60 minutes after each injection. In each sample LH and FSH levels were determined by radioimmunoassay. Melatonin treatment did not cause any significant change in basal or post-stimulation LH and FSH levels either in men or in post-menopausal women. These results do not support previous findings of an insults do not support previous findings of an inhibitory effect of melatonin on gonadotropin secretion, even with the same dose as the one used in this study. Further studies with higher doses of melatonin are needed to clarify the action of this drug on gonadotropin secretion.