Expression of the NR-1 subunit in Xenopus oocytes produces channels that respond to glutamate and are blocked by competitive and noncompetitive antagonists of the N-methyl-D-aspartate (NMDA) receptor. Ionic conductances through these channels are increased by coexpression with NR-2 receptor subunits. We have characterized the pharmacological properties of NMDA receptors assembled from combinations of subunits expressed in transfected cells, to determine the minimum subunit requirements for binding of competitive glutamate antagonists, glycine antagonists, and channel-blocking agents, as detected by ligand-binding experiments. Expression of NR-1a alone produced glycine antagonist binding, whereas the combination of NR-1a and NR-2A was needed to produce binding sites for glutamate antagonists and channel-blocking agents. These results suggest that functional NMDA receptors assemble from these subunits. However, differences in the pharmacological effects of NMDA and polyamines show that not all characteristics of native NMDA receptors are reproduced by this combination of subunits.