The effects of treatment with the selective kappa-opioid receptor agonist U-69593 upon cocaine-induced changes in locomotor activity and stereotypy were examined in rats. U-69593 (0.16 mg/kg s.c.) administered either acutely or chronically attenuated both the motor stimulant effect and stereotypy produced by an acute injection of cocaine (20 mg/kg i.p.). Daily cocaine treatment resulted in sensitization to both effects of cocaine. In contrast, no such sensitized responses were seen in animals which had received U-69593 either prior to or in conjunction with daily cocaine treatment. These data demonstrate that activation of kappa-opioid receptors attenuates the acute and chronic effects of cocaine on locomotor activity and stereotypy. Given the inhibitory effects ascribed to both exogenous and endogenous kappa-opioid agonists upon dopamine release in the mesolimbic dopaminergic system, it is suggested that this action may underlie the observed effects of U-69593 on cocaine-induced changes in locomotor activity and stereotypy.