In healthy young adults, the 24-hour profile of plasma growth hormone (GH) levels consists of stable low levels abruptly interrupted by bursts of secretion. In normal women, daytime GH secretory pulses are frequent. However, in normal men, a sleep-onset-associated pulse is generally the major or even the only daily episode of active secretion. Extensive evidence indicates the existence of a consistent relationship between slow-wave (SW) sleep and increased GH secretion. There is a linear relationship between the amount of SW sleep (measured by either visual scoring or spectral analysis of the EEG) and the amount of concomitant GH secretion. During ageing, SW sleep and GH secretion decrease exponentially and with the same chronology. Pharmacological stimulation of SW sleep results in increased GH release, and compounds that increase SW sleep may therefore represent a novel class of GH secretagogues.