The effect of systemic administration of the dopamine agonist apomorphine on plasma oxytocin concentration was studied in male rats by a specific radioimmunoassay. Apomorphine given subcutaneously in doses ranging from 80 to 480 micrograms/kg increased oxytocin levels in a dose-dependent manner. The minimal effective dose was found to be 80 micrograms/kg, which induced a 66% increase above basal values, while the maximal effect (210%) was seen with a dose of 240 micrograms/kg. The apomorphine effect was prevented by pretreatment with the DA D2-receptor blockers haloperidol (0.2 mg/kg i.p.) or (-) sulpiride (10 mg/kg i.p.) and, but only partially, with the DA D1-receptor blocker SCH 23390 (0.2 mg/kg s.c.). The present results suggest that hypothalamic dopamine has a facilitatory role on the release of oxytocin in male rats.