Alternative splicing of the mRNA encoding the N-methyl-D-aspartate (NMDA) receptor subunit NR1 changes the structural, physiologic, and pharmacologic properties of the resultant NMDA receptors. We used dual label immunocytochemistry and confocal microscopy to localize the four alternatively spliced segments of the NR1 subunit (N1, C1, C2, and C2') in rat striatal neurons. Striatofugal projection neurons and four populations of interneurons were studied. Projection neurons, which were identified by immunolabeling for calbindin and by retrograde tracing from the globus pallidus and the substantia nigra, were the only striatal neurons containing C1 segment immunoreactivity. Projection neurons were also C2 segment immunopositive, as were all other neuronal populations studied. Projection neurons were C2' segment immunonegative. In contrast, each of the interneuron types were labeled by the antibody to the C2' segment: nitric oxide synthase interneurons were labeled intensely, calretinin and parvalbumin neurons were labeled moderately strongly, and cholinergic neurons were also labeled but less strongly than the other types of interneurons. Parvalbumin interneurons showed distinct N1 segment immunolabeling, which was not found in other types of striatal neurons. Our results suggest that all striatal neurons studied synthesize NR1 subunit proteins, but the isoforms of the protein present in projection neurons and the several types of interneurons are distinct. This differential expression of NR1 isoforms may affect both neuronal function and selective vulnerability of neurons to injury.