D-AP5 is a selective antagonist of the NMDA receptor. D(−)-2-Amino-5-phosphonovaleric acid is described to antagonize amino acid-induced synaptic excitations, blocking L-aspartate and dorsal root-evoked spinal neuron excitation, without affecting cholinergic excitation induced by exogenous acetylcholine administration. Evoked electrical activity of neurons is suppressed by D(−)-2-Amino-5-phosphonovaleric acid antagonism of motoneuronal depolarizations at the NMDAR. The (+)-stereoisomer of D(−)-2-Amino-5-phosphonovaleric acid demonstrates considerably diminished NMDAR activity as compared to this (−)-isomer.