We examined the possibility of changes in gene expression of the NMDA receptor subunits after chronic antagonist treatment. Exposure of neurons to the NMDA antagonist D(-)-2-amino-5-phosphonopentanoic acid (AP-5) produced an increase in the levels of the R2B mRNA subunit. Concomitant exposure of neurons to AP-5 and NMDA reversed the upregulation. Chronic AP-5 treatment increased the R1 polypeptide, whereas no change was observed in the levels of mRNA encoding the R1 subunit. A more pronounced increase was observed in the R2A/B polypeptides. These data demonstrate that chronic treatment with NMDA antagonists selectively upregulates the NMDA receptor mRNAs and polypeptides. Furthermore, antagonist treatment produced a differential regulation of the R1, R2A, and R2B subunits in cortical neurons.