The relation between the pituitary-gonadal hormones' rhythm and sleep physiology in men is not fully elucidated. To examine whether the reproductive hormones are correlated with sleep architecture, we determined the nocturnal serum levels of testosterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) in six healthy young men. Serum hormone levels were obtained every 15 minutes from 1900 to 0700 hours with simultaneous polysomnographic sleep recordings. Hourly testosterone levels were lowest when subjects were awake (1900-2200 hours) than during sleep (2300-0700 hours). Testosterone nocturnal rise antedated the first REM by about 90 minutes. The rise in testosterone levels was slower when REM latency was longer. Mean nocturnal testosterone levels did not correlate with the number of rapid eye movement (REM) episodes. Also, pre-non-REM (NREM) testosterone levels were higher as compared with the pre-REM periods and lower during the first NREM period as compared with other nocturnal NREM periods. Serum LH levels disclosed a nocturnal rise that preceeded a similar rise in testosterone by about an hour. We conclude that in young adult men, testosterone levels begin to rise on falling asleep, peak at about the time of first REM, and remain at the same levels until awakening.