A withdrawal syndrome was precipitated by naloxone in morphine-dependent rats injected with 5,7-dihydroxytryptamine (5,7-DHT) in the ventromedial tegmentum (VMT) at the level of the nucleus interpeduncularis. 5,7-DHT, which markedly depleted 5-hydroxytryptamine (5-HT) in the forebrain but not in the brainstem, significantly reduced jumping in abstinent rats with no significant effect on other withdrawal signs. The effect of morphine 10 mg kg-1 on responses on the hot plate was unchanged in 5,7-DHT-treated rats. The findings suggest that 5-HT in the forebrain is selectively involved in the jumping of morphine-abstinent rats.