The role of endogenous opiates in the control of the secretion of oxytocin in the basal state in healthy male volunteers was investigated with the opiate antagonist naloxone. There was no change in plasma oxytocin levels, assessed for a 120 min period following the intravenous administration of naloxone (10 mg). Although there was no effect of opiate receptor blockade with naloxone in this basal situation, further studies are needed to evaluate the possible role of opioid regulation of oxytocin during lactation and parturition.