The role of endogenous opioid receptors on anterior pituitary hormone secretion was evaluated by the administration of the opioid receptor antagonist naloxone. The infusion of naloxone (8 mg iv followed by 4mg/h for 3 h) did not alter basal growth hormone (GH), prolactin (Prl) and thyrotrophin (TSH) secretion but produced a significant rise in cortisol and gonadotrophins in normal man. The infusion of the opiate antagonist appeared to increase the rate and amplitude of luteinizing hormone (LH) pulsatility. Naloxone pre-medication (10 mg iv 30 min before testing) did not alter the pituitary response to TRH and LRH stimulation. These results demonstrate that naloxone can modify basal anterior pituitary hormone secretion and strongly suggest an endogenous opioid modulation of some of these hormones.