We studied the role of nocturnal illuminance as a possible regulatory factor in the production and secretion of reproductive hormones. Adult male rats were kept under artificial lighting conditions (LD 12/12 hours) where the daily illuminance was 520-550 lx and the nocturnal illuminance 0, less than 0.1, or 1-1.5 lx. After a 7 day adaptation period, the pineal contents of melatonin, the serum levels of LH, FSH, and prolactin, and the pituitary contents of these hormones were measured by radioimmunoassay in samples taken 4 and 7 hours after the onset of light and onset of darkness. When the rats were kept in full darkness or under the minimal nocturnal iluminance (less than 0.1 lx), the patterns of pineal melatonin were equal. When nocturnal illuminance was 1-1.5 lx, the pineal melatonin contents at night were lower than in complete darkness. However, this illuminance level did not totally abolish the nocturnal increase of melatonin synthesis. The serum levels of LH, FSH, and prolactin were all affected by the increase of nocturnal illuminance so that the difference between the daytime and nighttime levels increased (higher values during the night). The variation of nocturnal illuminance did not change the pituitary contents of gonadotropins. The effects on pituitary prolactin were negligible as well. The results are interpreted to support the concept that nocturnal illuminance levels may regulate the secretion of the studied hormones in male rats.