In recent years, the distribution of dopamine receptor subtypes among the principal neurons of the neostriatum has been the subject of debate. Conventional anatomical and physiological approaches have yielded starkly different estimates of the extent to which D₁ and D₂ class dopamine receptors are colocalized. One plausible explanation for the discrepancy is that some dopamine receptors are present in physiologically significant numbers, but the mRNA for these receptors is not detectable with conventional techniques. To test this hypothesis, we examined the expression of DA receptors in individual neostriatal neurons by patch-clamp and RT-PCR techniques. Because of the strong correlation between peptide expression and projection site, medium spiny neurons were divided into three groups on the basis of expression of mRNA for enkephalin (ENK) and substance P (SP). Neurons expressing detectable levels of SP but not ENK had abundant mRNA for the D_1a receptor. A subset of these cells (∼50%) coexpressed D₃ or D₄receptor mRNA. Neurons expressing detectable levels of ENK but not SP had abundant mRNA for D₂ receptor isoforms (short and long). A subset (10–25%) of these neurons coexpressed D_1aor D_1b mRNAs. Neurons coexpressing ENK and SP mRNAs consistently coexpressed D_1a and D₂ mRNAs in relatively high abundance. Functional analysis of neurons expressing lower abundance mRNAs revealed clear physiological consequences that could be attributed to these receptors. These results suggest that, although colocalization of D_1a and D₂ receptors is limited, functional D₁ and D₂ class receptors are colocalized in nearly one-half of all medium spiny projection neurons.