The NMDA subtype of excitatory glutamate receptor is a multisubunit, fast-acting, ligand-gated cation channel with a high permeability for Ca2+ . NMDA receptors have been shown to be of importance in physiological processes, such as long-term potentiation, and also in pathophysiological conditions, including chronic neuronal degeneration and acute excitotoxity (1 ). Molecular cloning has identified five genes encoding two types of NMDA receptor subunits, the NR1 and NR2A-D subunits. Additionally, the NR1 subunit gene undergoes alternative splicing to yield eight variant forms, thus providing further heterogeneity. The quaternary structures of NMDA receptors are not known, but most are believed to be heteromeric complexes comprising multiple copies of both NR1 and NR2 subunits in as-yet unknown stoichiometries (summarized in 1 ). Since the absolute polypeptide compositions of native NMDA receptors are unknown, a convenient model system in which to study the properties of defined cloned NMDA receptor subtypes and thus to compare them with their in vivo counterparts is their expression in mammalian cells. This permits the pharmacological, functional and biochemical characterization of either a single type of NMDA receptor subunit or coexpression of combinations of NR1 and NR2 subunit genes (e.g., 2 –4 ).