The effect of antioxidants and reducing agents on glutamate-induced cytotoxicity was examined using PC12 cells. The antioxidants vitamin E, idebenone, and selegiline protected cells against the cytotoxicity observed 24 h after exposure to 0.5 or 10 mM glutamate, as determined by lactate dehydrogenase leakage, even when added 3 h after glutamate. The reducing agents, glutathione (GSH) and dithiothreitol (DTT), also provided protection against the cytotoxicity of glutamate. Preincubation of PC12 cells with the antioxidants mentioned above, or the incubation with those antioxidants after exposure to glutamate for 3 h, prevented the reduction of viability caused by glutamate. Cystine uptake was inhibited by exposure of cells to glutamate, as determined by L-[35S]-cystine uptake. Incubation of cells with 0.5 or 10 mM glutamate caused a marked decrease in cellular GSH levels, not prevented by antioxidants. The activity of GSSG reductase was decreased by glutamate and this inhibition was reverted in the presence of the reducing agents GSH and DTT. These results indicate that glutamate toxicity on PC12 cells results from the inhibition of cystine uptake with consequent GSH depletion and oxidative stress, suggesting that antioxidants may reduce the cellular damage in pathologic conditions associated with excessive glutamate release.