Endogenous opioid peptides inhibit secretion of oxytocin during dehydration, hemorrhage and parturition and attenuate release of vasopressin by tail electroshock. Diverse agents were used to stimulate the hypothalamo-neurohypophysial system to investigate the hypothesis that if oxytocin (or vasopressin) release were inhibited by opioid peptides regardless of the stimulus, the site of opiate action may be in the final common pathway (i.e. the magnocellular neuron) or on pituicytes in the neural lobe. Using male Sprague-Dawley rats, we therefore investigated the effect of an opiate receptor antagonist, naloxone (5 mg/kg s.c), on the plasma concentrations of oxytocin and vasopressin elevated by various pharmacologic stimuli, including histamine (10 mg/kg i.p.), nicotine (0.15 or 1.5 mg/kg i.p.), isoproterenol (30 or 120 ug/kg i.m.) and increased [NaCl] in cerebrospinal fluid (CSF; 10 µl artificial CSF containing 1 MNaCl i.v.t.). Control animals received saline (0.85%) or artificial CSF (containing 0.16 M NaCl). Animals were decapitated 60 s (†[NaCΓJ in CSF) or 10 min after the stimulus or vehicle. Vasopressin and oxytocin were extracted from plasma and quantified by RIA. The concentrations of oxytocin and vasopressin in plasma were elevated (p < 0.05) by histamine, isoproterenol (30 and 120 αg/ kg), ↑[NaCl] in CSF, and nicotine at the higher (1.5 mg/kg) but not lower (0.15 mg/kg) dose. Naloxone increased further (p<0.05) the concentration of oxytocin in plasma after histamine, nicotine (0.15 and 1.5 mg/kg), isoproterenol (30 and 120 µg/kg) and ↑[NaCl] in CSF. Naloxone also increased (p<0.05) oxytocin concentration in controls receiving CSF or saline. Vasopressin concentration was unaffected by naloxone in either controls or after any of the stimuli. We conclude that endogenous opioid peptides inhibit release of oxytocin in vivo by an effect on the final common pathway, i.e. the magnocellular neuron or on pituicytes in the neural lobe.