The purpose of our study was to examine the effects of D1-and D2-dopamine receptors blockade on the changes in the ventricular content of catecholamines in rats withdrawn from morphine. Rats were given morphine by subcutaneous (s.c.) implantation of morphine pellets for 5 days. On the eighth day, morphine withdrawal was induced by s.c. administration of naloxone (1 mg/kg), and rats were killed 30 min later. Pretreatment with SCH 23390 (dopamine D1, D5 receptor antagonist) 15 min prior to naloxone administration suppressed some the behavioural signs of morphine withdrawal, whereas eticlopride (dopamine D2, D3, D4 receptor antagonist) did not. In addition, biochemical analysis indicate that SCH 23390 completely abolished the withdrawal-induced increase in noradrenaline and dopamine turnover in the right ventricle. By contrast, eticlopride did not block the hyperactivity of catecholaminergic neurons in the heart during morphine withdrawal. These data suggest that the hyperactivity of catecholaminergic neurons in the heart during morphine withdrawal is dependent upon D1 dopamine receptor activation. In addition, our results exclude the involvement of D2 dopamine receptors.