Immunohistochemical double-label techniques were used to study the localization of DARPP-32, a phosphoprotein that is enriched in neurons possessing members of the D₁ subfamily of dopamine receptors, in several different types of striatal neurons in the rat basal ganglia. The vast majority (94.1%) of striatonigral projection neurons (the vast majority of which contain substance P), identified by retrograde labeling with fluorogold, were observed to contain DARPP-32. Similarly, the vast majority of striatopallidal projection neurons (87.7%), identified by immunofluorescence labeling for enkephalin (ENK), were found to label for DARPP-32. In contrast, cholinergic and neuropeptide Y-containing striatal interneurons were never observed to contain DARPP-32. These results suggest that essentially all major types of striatal medium spiny projection neurons may possess members of the D₁ subfamily of dopamine receptors, but that striatal local circuit neurons do not possess members of the D₁ subfamily of receptors.