The hyperactivity induced by injection of the dopamine (DA) agonist apomorphine into the nucleus accumbens of rats was dose dependently inhibited by intra-accumbal pretreatment with cholecystokinin (CCK-8) (ED50, 0.34 ng). Local treatment with the opioid antagonist naloxone antagonized this inhibitory action of CCK-8 (ED50, 18 ng), but did not change the blocking effect of haloperidol on the apomorphine-induced hyperactivity. These data show that endogenous opioids are concerned in certain interactions between CCK-8 and DA systems.