The role of melatonin in the regulation of reproduction in humans is unknown. We conducted a 6-month, double-blind, crossover study of a daily treatment dose of 3 mg melatonin or placebo given orally at 1700 hours in 8 healthy men. Semen quality (concentration, motility, and morphology), serum and seminal plasma 17-beta-estradiol (E(2)), testosterone, melatonin, and serum gonadotropin levels were determined every 3 months throughout the study. In 6 men, there was no change in semen quality or in serum and seminal plasma hormone levels during the study period. In 2 men, during the melatonin treatment period, sperm concentration decreased to 3 x 10(6)/mL and 12 x 10(6)/mL, and motility declined to 32% and 30%. These coincided with a decline in seminal plasma and serum E(2) levels and with an increase in testosterone:E(2) ratios. Six months after the cessation of melatonin, sperm concentration and motility were normal in 1 man but remained abnormal in the other one with a still elevated testosterone:E(2) ratio. Serum gonadotropin levels were unchanged during the study in all 8 men. Our preliminary observations suggest that long-term melatonin administration is associated with decreased semen quality in a number of healthy men, probably through the inhibition of aromatase at the testicular level.