N-methyl-D-aspartate (NMDA) activates a class of excitatory amino acid receptor involved in a variety of plastic and pathological processes in the brain. Quantitative study of the NMDA receptor has been difficult in mammalian neurons, because it usually exists with other excitatory amino acid receptors of overlapping pharmacological specificities. Xenopus oocytes injected with messenger RNA isolated from primary cultures of rat brain have now been used to study NMDA receptors. The distinguishing properties of neuronal NMDA receptors have been reproduced in this amphibian cell, including voltage-dependent block by magnesium, block by the NMDA receptor antagonist D-2-amino-5-phosphonovaleric acid, and potentiation by glycine. This preparation should facilitate the quantitative study of the regulation of NMDA receptor activation and serve as a tool for purification of the encoding messenger RNA.