L-Glutamate neurotoxicity at the N-methyl-D-aspartate (NMDA) receptor was characterized in cultured cerebellar granule cells. When deprived of glucose for 40 min, these cells were killed by 20-60 microM L-glutamate. However, the neurons were resistant to glutamate at concentrations as high as 5 mM when glucose and Mg2+ were present throughout. Both competitive and non-competitive NMDA receptor antagonists completely blocked neurotoxicity due to glutamate and other NMDA receptor agonists. CPP [+/-)-3-(2-carboxypiperazin-4-yl)-prophyl-1-phosphonic acid) was the most effective competitive antagonist with full protection at 100 microM while MK-801 [+/-)-10,11-dihydro-5-methyl-5H-dibenzo[a,d]-cyclohepten-5,10-imin e) was the most effective non-competitive antagonist with full protection at 20 nM. Other antagonists with higher selectivity for other subtypes of glutamate receptors were ineffective. We conclude that glutamate toxicity in energy-deprived cerebellar granule cells is mediated by NMDA receptors. Results are discussed in terms of an hypothesis offering an explanation for the transition of glutamate from neurotransmitter to neurotoxin which emphasizes the responsiveness of the receptor to agonists rather than focusing on the presence of high concentrations of agonist.