The role of oxygen-derived free radicals, superoxide in particular, in the pathogenesis of neuronal cell death induced by glutamate was studied using cultured cortical neurons from transgenic mice overexpressing human copper-zinc-superoxide dismutase. Primary cortical neuron cultures were developed from 15-day-old fetuses of both transgenic mice and their normal littermates. Both human copper-zinc-superoxide dismutase and host mouse copper-zinc-superoxide dismutase activities in cultured neurons were identified by native gel electrophoresis followed by nitroblue tetrazolium staining. Cultured neurons grown for 10-12 days in vitro were exposed briefly to 0.5 mM glutamate for 5 minutes, followed by biochemical and morphological examinations at 2, 4, and 24 hours. Our data have demonstrated that glutamate neurotoxicity is significantly reduced in transgenic neurons at 2 and 4 hours following exposure to glutamate, as measured by the efflux of lactate dehydrogenase, the 3-O-methyl glucose space, and by phase-contrast and bright-field trypan blue staining. These data indicate that transgenic neurons containing twofold to threefold the normal amount of copper-zinc-superoxide dismutase activity as the result of expression of the human copper-zinc-superoxide dismutase transgene are protected against glutamate neurotoxicity in vitro. Our results suggest that oxidative stress, at least in part, plays an important role in the biochemical pathways amplifying N-methyl-D-aspartate receptor-mediated neurotoxicity.