The pathophysiology of hypoxic neuronal death, which is difficult to study in vivo, was further defined in vitro by placing dispersed cultures of rat hippocampal neurons into an anoxic atmosphere. Previous experiments had demonstrated that the addition of high concentrations of magnesium, which blocks transmitter release, protected anoxic neurons. These more recent experiments have shown that gamma-D-glutamylglycine (DGG), a postsynaptic blocker of excitatory amino acids, was highly effective in preventing anoxic neuronal death. DGG also completely protected the cultured neurons from the toxicity of exogenous glutamate (GLU) and aspartate (ASP). In parallel physiology experiments, DGG blocked the depolarization produced by GLU and ASP, and dramatically reduced EPSPs in synaptically coupled pairs of neurons. These results provide convincing evidence that the synaptic release of excitatory transmitter, most likely GLU or ASP, mediates the death of anoxic neurons. This result has far-reaching implications regarding the interpretation of the existing literature on cerebral hypoxia. Furthermore, it suggests new strategies that may be effective in preventing the devastating insults produced by cerebral hypoxia and ischemia in man.