We have used cell-attached patch-clamp electrophysiology to characterize the activation and distribution of an 85 pS K+ channel on freshly dissociated rat striatal (caudate-putamen) neurons. In recordings from 643 cells, openings of this channel showed an absolute dependence on the presence of dopamine or the D2-like dopamine receptor agonist quinpirole in the cell-attached patch pipette, but were never seen when the D2 antagonist domperidone was applied along with quinpirole, or in the absence of drug. This channel displayed inward rectification at depolarized membrane potentials, but its activation was otherwise voltage insensitive. It was largely restricted to a subset of dissociated cells with diameters > or = 10 microns, with channel openings seen in about 25% of patches. When present, there were typically multiple channels per patch. Cells of this size were immunocytochemically stained for neuron-specific enolase but not glial fibrillary acidic protein; about 40% were also labeled for gamma-amino butyric acid (GABA) and about 60% for NADPH diaphorase, with GABAergic cells displaying a shape most similar to that of cells expressing the channel. A large number of distinct types of other channels were also present, comprising inwardly rectifying channels of 5–35 pS conductance and voltage-activated channels of 100–250 pS, but the frequencies of occurrence and fractional open times of these channels were independent of the presence or absence of dopaminergic agonists. Thus, the 85 pS K+ channel uniquely requires activation by a D2-like dopamine receptor on rat striatal neurons, and is selectively expressed by a subset of these cells, which are most likely to be GABAergic neurons.