Sulfated cholecystokinin octapeptide (CCK) potentiates dopamine-induced hyperlocomotion in the nucleus accumbens of the rat. Immunocytochemical evidence has shown a topographical distribution of terminals containing both CCK and dopamine (DA), within the medial posterior nucleus accumbens. Seven sites within the nucleus accumbens were cannulated and tested for the ability of CCK to enhance the behavioral effects of DA. Close agreement was found between the anatomical sites of CCK-DA coexistence, and the anatomical sites at which CCK potentiated DA-induced hyperlocomotion. Behaviorally inactive sites were found primarily in the anterior nucleus accumbens, where DA-containing terminals do not contain CCK.