In order to evaluate the role of the pineal gland in human reproduction, day- and night-time concentrations of serum melatonin, FSH, LH and prolactin were measured by radioimmunoassays on various days of the menstrual cycle during summer (average daylight 22 h) and winter (daylight 5 h) in healthy females (n = 12) from northern Finland (65 degrees N). A multifactorial analysis of variance showed that, in addition to the well-established increases of gonadotropins at midcycle and melatonin and prolactin at night, there was a significant effect of season on the serum levels of melatonin and LH. Night-time serum melatonin levels on cycle days 2 and 10 were 27% and 49% (P less than 0.05 and less than 0.01) higher in winter than in summer. Night-time serum LH levels at midcycle were 76% (P less than 0.05) higher in summer than in winter. There were no significant effects of season on the serum levels of FSH, prolactin, day-time melatonin or LH outside the mid-cycle. Neither were there any significant effects of the day of the menstrual cycle on the serum melatonin levels. It is possible that in winter the high levels of melatonin in the follicular phase have an inhibitory effect on the serum LH levels. In summer the melatonin levels are lower and perhaps less inhibitory on the secretion of LH, resulting in the stimulation of the reproductive competence in human females.