Systemic administration of growth hormone-releasing hormone (GHRH) has been found to improve human sleep in previous studies. Here we examined effects of GHRH on endocrine function and sleep after intranasal administration, a method which based on previous studies appears to enable a direct effect of peptides on brain function. Also, it was hypothesized that elderly humans displaying deficient GH release and sleep, benefit from GHRH administration more than young subjects. A study was performed according to a double-blind cross-over design. Each of 12 young and 11 old healthy men were intranasally administered with 300 micrograms GHRH (vs. placebo) 30 min before bedtime at 23:00 h. Sleep was recorded polysomnographically until 07:00 h and blood was collected in 15 min intervals for determination of cortisol and GH. Apart from the well-known age-related changes of hormonal secretion and sleep, intranasal GHRH reduced cortisol nadir concentrations in the beginning of sleep (P < 0.05), and also reduced the sleep-induced elevation in GH concentrations during early sleep. Moreover, results indicated that after intranasal administration GHRH increased rapid-eye-movement (REM) sleep and slow wave sleep (SWS), with this influence concentrating on the second half of sleep time. Effects of GHRH did not depend on the subject's age. We conclude that there is a coordinate influence of intranasal GHRH on the central nervous regulation of sleep processes and of hypothalamic-hypophysiotropic secretory activity in both young and elderly men. The effects may mimic the dual neuronal and endocrine function of hypothalamic GHRH activity.