The effect of the dopamine (DA) agonist, apomorphine, on oxytocin concentrations in the hypothalamus, hippocampus, septum and plasma was studied in male rats. Apomorphine dose dependently increased the concentration of oxytocin in the plasma and hippocampus, the minimal effective dose being 80 micrograms/kg s.c., which induced a 65% increase in plasma and a 45% increase in the hippocampus. The maximal effect (210 and 125% above controls) was induced with 240 micrograms/kg s.c. In contrast, there was a significant decrease (32%) in the oxytocin concentration in the hypothalamus, but only after the highest doses of apomorphine, while no change was found in the septum. The apomorphine effect in the hippocampus and hypothalamus was prevented by the mixed DA D-1/D-2 receptor blocker, haloperidol (0.3 mg/kg i.p.), and by the DA D-2 receptor blocker, (-)-sulpiride (20 mg/kg i.p.), but not by the DA D-1 receptor blocker, SCH 23390 (0.2 mg/kg s.c.). Similar effects were found in plasma, although SCH 23390 inhibited the apomorphine effect by 45%. Our results suggest that apomorphine stimulates oxytocinergic transmission in male rats and provide biochemical support for the hypothesis that a DA-oxytocin link exists in the central nervous system.