The action of cholecystokinin octapeptide (CCK-8) on rat neostriatal dopamine (DA) D2 receptors was evaluated in membrane binding experiments. 0.1 nM of CCK-8 increased the Kd value of the D2 agonist [3H]N-propylnorapomorphine (NPA) binding sites by 42%. The CCKB antagonist PD134308 blocked this action. Kinetic analysis demonstrated that this effect of CCK-8 was related to a reduction by 45% of the association rate constant of [3H]NPA. In contrast, 1 nM of CCK-8 decreased the KH and the KL values of DA for the D2 antagonist [3H]raclopride binding sites by 56% and 50%, respectively. Both the CCKA antagonist L364718 and the CCKB antagonist PD134308 blocked this effect. The D1 antagonist SCH23390 counteracted the CCK-8 induced decrease in the KH and the KL values of DA, and allowed 1 nM of CCK-8 to produce a significant increase in the IC50 value of NPA for the [3H]raclopride binding sites. These results indicate that CCK-8 can reduce the affinity of the neostriatal D2 agonist binding sites, but increase the affinity of D2 receptors for DA. D1 receptors may exert a switching role in the modulation of the neostriatal D2 receptors by the CCK receptors.