The effect of naloxone, a specific antagonist of opioid peptides, on plasma, GH, PRL, and cortisol responses to insulin-induced hypoglycemia was studied in five healthy male subjects. The iv administration of regular insulin (0.15 U/kg) led to similar degrees of hypoglycemia on control and experimental days. Plasma GH, PRL, and cortisol levels rose significantly in response to insulin-induced hypoglycemia. A 2-h infusion of naloxone (0.8 mg/h) started 30 min before insulin injection did not alter either basal hormone levels or the hormone responses to insulin-induced hypoglycemia. These results suggest that endogenous opioid peptides do not play a major role in GH, PRL, or cortisol secretion induced by insulin hypoglycemia.