To evaluate the role of endogenous opioid peptides in prolactin (Prl), growth hormone (GH) and cortisol neuroregulation, 50 mg of the opiate antagonist naloxone was infused over 24 h to 6 normal male volunteers. An additional naloxone dose (5 mg) was given i.v. as a bolus injection at 2000 h. Blood specimens were collected hourly by means of a portable constant withdrawal pump. Naloxone failed to alter 24 h secretion of GH and Prl. The sleep-related GH and Prl rise was also unaffected by the opiate blocker. Naloxone failed to alter the circadian rhythm of cortisol and its 24 h concentration. A major role of opiate receptors in spontaneous GH, Prl and cortisol secretion in man was not supported.