The hypothesis of N-methyl-D-aspartate (NMDA) receptor hypofunction in schizophrenia was initially based on observations that blockade of the NMDA subtype of glutamate receptor by noncompetitive antagonists, such as phencyclidine and ketamine, can lead to clinical symptoms similar to those present in schizophrenia. Recently, glutamate has also been implicated in the pathophysiology of the mood disorders. As impaired NMDA receptor activity may be the result of a primary defect in the NMDA receptors themselves, or secondary to dysfunction in the protein complexes that mediate their signaling, we measured expression of both NMDA subunits and associated postsynaptic density (PSD) proteins (PSD95, neurofilament-light (NF-L), and SAP102) transcripts in the dorsolateral prefrontal cortex in subjects with schizophrenia, bipolar disorder, major depression, and a comparison group using tissue from the Stanley Foundation Neuropathology Consortium. We found decreased NR1 expression in all three illnesses, decreased NR2A in schizophrenia and major depression, and decreased NR2C in schizophrenia. We found no changes of NR2B or NR2D. Receptor autoradiography revealed no alterations in receptor binding in any of the illnesses, indicating no change in total receptor number, but taken with the subunit data suggests abnormal receptor stoichiometry. In the same subjects, PSD95 was unchanged in all three illnesses, while reduced NF-L expression was found in schizophrenia, especially in large cells of layer V. SAP102 expression was reduced in bipolar disorder restricted to small cells of layer II and large cells of layer III in bipolar disorder. These alterations likely reflect altered signaling cascades associated with glutamate-mediated neurotransmission within specific cortical circuits in these psychiatric illnesses. © 2008 Nature Publishing Group All rights reserved.