To investigate the effect of melatonin(Mel) on follicle-stimulating hormone(FSH), luteinizing hormone(LH), estradiol(E 2) and ovulation, 100 healthy women were divided into 5 groups to take 0mg, 12mg, 30mg, 75mg, and 300mg Mel per day, respectively, for 3 months. The levels of FSH, LH, E 2 were determined by enzyme-linked immunosorbent assay(ELISA) before and after medication, the development of dominant follicle was detected by ultrasound. We have found that the levels of FSH, LH and E 2 were all lower and the dominant follicle was smaller than those of the control (0mg) group at 3 months. In this study we demonstrated the direct involvement of melatonin in modulation of the female reproductive system in a dose-dependent manner, and it may be put into effect through melatonin receptors in human hypothalamus-pitutary-ovary cells.