This report deals with the effect of cholecystokinin octapeptide (CCK-8) on the regulation of the behavior stimulated by dopaminergic drugs. Bilateral injection of CCK-8 (1 microgram per side) into the nucleus caudatus significantly reduced the locomotor hyperactivity induced by methamphetamine. Stereotyped sniffing and yawning occurred after intrastriatal administration of apomorphine (20 micrograms per side). Injections of CCK-8 into the nucleus caudatus completely inhibited the sniffing, but did not affect the yawning induced by apomorphine. It also had no effect on the basal dopamine (DA) level or the methamphetamine-induced DA level in the striatum. These results suggest that the injection of CCK-8 into the nucleus caudatus selectively inhibited the function of the dopaminergic system in the striatum, and blocked post-synaptic DA receptors.