The neuroexcitotoxin kainate has been used as a selective lesioning agent to model the etiology of a number of neurodegenerative disorders. Although excitotoxins cause susceptible neurons to undergo prolonged or repeated depolarization, the proximate metabolic pathology responsible for neuronal necrosis has remained elusive. We report here that kainate-induced death of cerebellar neurons in culture is prevented by inhibiting the enzyme xanthine oxidase, a cellular source of cytotoxic superoxide radicals (O2-.). Moreover, neurons are also protected from excitotoxin-induced death by the addition to the culture medium of either superoxide dismutase or mannitol, which scavenge superoxide and hydroxyl radicals, respectively, or serine protease inhibitor, which forestalls formation of xanthine oxidase. These findings indicate that excitotoxin-induced neuronal degeneration is mediated by superoxide radicals generated by xanthine oxidase, a mechanism partially analogous to that proposed for tissue damage seen upon reperfusion of ischemic tissues.