NMDA receptors are a subclass of excitatory, ionotropic L-glutamate neurotransmitter receptors. They are.heteromeric, integral membrane proteins being formed by the assembly of the obligatory NR1 subunit together with modulatory NR2 subunits of which four different types, NR2A - NR2D, have been described. This results in a heterogenous population of receptor proteins with distinct pharmacological and biophysical properties thus yielding potential for the development of NMDA receptor subtype-selective therapeutic agents. Anti-NMDA receptor subunit antibodies have been generated and used in immunoprecipitation or immunoaffinity purification studies to determine the in vivo subunit complements of NMDA receptors. This article summarizes knowledge on the subunit compositions of NMDA receptors based on these approaches together with the current status of NMDA receptor subunit stoichiometry and hence quaternary structure. of native NMDA receptors.