NMDA receptors are composed of proteins from two families: NMDAR1, which are required for channel activity, and NMDAR2, which modulate properties of the channels. The mRNA encoding the NMDAR2D subunit has a highly restricted pattern of expression: in the forebrain, it is found in only a small subset of cortical, neostriatal and hippocampal neurons. We have used a quantitative double-label in situ hybridization method to examine the expression of NMDAR2D mRNA in neurochemically defined populations of neurons. In the neostriatum, NMDAR2D was expressed by the interneuron populations marked by preprosomatostatin (SOM), the 67-kDa form of glutamic acid decarboxylase (GAD67), parvalbumin (PARV), and choline acetyltransferase (ChAT) mRNAs but not by the projection neurons expressing β-preprotachykinin (SP) or preproenkephalin (ENK) mRNAs. In the neocortex, NMDAR2D expression was observed in only a small number of neurons, but these included almost all of the SOM-, GAD67-, and PARV-expressing interneurons. In the hippocampus, NMDAR2D was not present in pyramidal or granule cells, but was abundant in SOM-, GAD67-, and PARV-positive interneurons. NMDAR2D expression appears to be a property shared by interneurons in several regions of the brain. The unique electrohysiological characteristics conveyed by this subunit, which include resistance to blockade by magnesium ion and long channel offset latencies, may be important for the integrative functions of these neurons. NMDAR2D-containing receptor complexes may prove to be important therapeutic targets in human disorders of movement. In addition, the presence of NMDAR2D subunits may contribute to the differential vulnerability of interneurons to excitotoxic injury.