Cholecystokinin octapeptide sulfate (CCK) injected bilaterally into the nucleus accumbens significantly potentiated dopamine-induced hyperlocomotion and apomorphine-induced stereotypy. CCK had no effect alone on locomotion or stereotypy, suggesting that this peptide acts as a neuromodulator. CCK did not potentiate dopamine or apomorphine when injected into the caudate nucleus, where CCK and DA are located in separate neurons, indicating that CCK potentiation of dopamine is specific for the mesolimbic pathway, where CCK and dopamine co-exist in the same neuron. Unsulfated CCK had no effect alone or in conjunction with dopamine or apomorphine, suggesting that the facilitory effect of CCK on dopamine-mediated behaviors is specific for the sulfated form of CCK.