Cholecystokinin octapeptide sulfate (CCK-S) is a neuropeptide that is co-localized with dopamine (DA) in some neurons of the ventral tegmental area (VTA). A functional role for this peptide/monoamine co-localization has not been firmly established; however, behavioral and in vivo electrophysiological studies indicate that CCK-S modifies the action of DA in some brain areas. A brain slice preparation of the rat VTA was developed in order to examine primary effects of CCK-S on DA-containing neurons, and to determine whether CCK-S modulates the inhibitory action of DA on these neurons. Spontaneously active DA neurons of the VTA were identified on the basis of their characteristic spike waveforms and firing rate as determined with extracellular recording techniques. These cells were inhibited by perfusion with DA in a dose-dependent, sulpiride-reversible manner. CCK-S produced brief excitatory increases in firing rate in 83% of these cells tested. This excitation was dose-dependent, and the excitatory responses frequently diminished even in the continued presence of CCK-S. Prior administration of CCK-S to these cells markedly potentiated DA-induced inhibition of spontaneous firing; the magnitude of this effect ranged from a 24 to 376% increase in the inhibitory response. This CCK-induced potentiation of DA inhibition was not blocked by low calcium, high magnesium superfusion medium, indicating that this effect is a direct consequence of a postsynaptic action on the VTA neurons from which recordings were made. These results suggest that co-localized CCK-S may significantly affect neuronal sensitivity to synaptically released DA.