The role of melatonin in human reproductive physiology remains obscure. Previous investigations of the nocturnal melatonin secretory pattern during the human menstrual cycle have yielded conflicting results and have been limited by incomplete characterization of the pertinent features of the circadian pattern such as magnitude, duration, or circadian position of the nocturnal increase, by incomplete assessments of ovarian secretion and menstrual cycle phase, or by both. In the present study, the circadian melatonin profile was characterized by obtaining plasma samples at hourly intervals from 10.00 to 17.00 h and at half-hourly intervals from 17.00 to 10.00 h in 10 women during four prospectively estimated menstrual phases: early follicular, late follicular, midluteal, and late luteal. Menstrual phases and ovarian function were confirmed by obtaining daily serum samples for LH, FSH, estradiol, and progesterone during the entire menstrual cycle. Eight women had ovulatory cycles with luteal lengths of greater than 11 days and with determinations of the circadian melatonin pattern during each menstrual phase. Despite gonadal steroid excursions characteristic of ovulatory cycles, consistent variations in plasma melatonin patterns by menstrual phase were not found. Daytime levels were at or below 43 pmol/l in all women and all but 2 women displayed prominent nocturnal elevations. While changes in the circadian pattern of melatonin secretion immediately preceding the LH surge cannot be excluded, the lack of consistent variation by menstrual cycle phase and the absence of clear circadian variation found in 2 women suggests that the secretory profile of melatonin is unaltered by fluctuations in gonadal steroids and that melatonin secretion may not be necessary for menstrual cyclicity to occur.