The patterns of plasma melatonin, gonadotropins, sex steroids and prolactin were studied in anovulatory infertile females undergoing ovulation induction with hMG/hCG. Melatonin levels were found to fluctuate during the menstrual cycle of these subjects with a nadir at mid-cycle and peak occurring at the early follicular/late luteal phases of the cycle (p < 0.05). Melatonin correlated negatively with estradiol during the follicular phase (r=-0.5180, p < 0.05) and positively with LH (5 + 0.6321, p < 0.05) in the luteal phase, respectively. Correlational analyses by partial and multiple correlations suggest that the effects of estradiol and LH on melatonin in the follicular phase are interdependent whereas the effect of LH on melatonin in the luteal phase is independent of the effects of other hormones. The results suggest that hormonal interactions and phases of the cycle are important variables contributing to the fluctuations in melatonin levels during the menstrual cycle.