There is limited information on the relation between age-related changes in pituitary-gonadal function and sleep physiology in older men. In this cross-sectional study of 67 healthy volunteers free from sleep complaints, aged 45-75 yr, we (1) assessed peak fluctuations and hourly variations in plasma testosterone, LH and prolactin sampled every 20 min during sleep, and (2) explored the relation between sleep parameters, respiratory and periodic leg movement disturbances, and circulating concentrations of the aforementioned hormones. We hypothesized that alterations in sleep architecture or presence of sleep disorders may contribute to hormonal variations in aging subjects. Bioavailable testosterone decreased and LH increased with age, but there were no differences in total testosterone and prolactin and no major changes in peak hormonal release. There were positive associations, independent from the age effect, between sleep efficiency, decreased latency to onset of REM activity, and number of REM episodes, and circulating testosterone. Sleep-disordered breathing significantly increased with age. The degree of respiratory disturbance was associated with decreased overnight plasma bioavailable testosterone, but this relationship lost its significance after age-adjustment. The findings demonstrate a decrease in gonadal function during sleep in healthy aging men and suggest that changes in sleep efficiency and architecture are associated with alterations in gonadal activity in these older individuals.