Induction of free radicals and succeeding cascade reactions of lipid peroxidation are known to cause neuronal damage in cases of cerebral ischemia. We studied whether hypoxic stress in vitro induces the formation of these cytotoxic materials, and whether superoxide dismutase (SOD) protects cultured neurons from hypoxic stress. We used human recombinant SOD (rSOD), which is structurally homologous to native human SOD, the only exception being that the N terminus is not acetylated. First, we detected free radicals by means of nitroblue tetrazolium (NBT) and assured that hypoxic stress to the cultured neurons induced the formation of peroxidation materials. Immediately after the hypoxic stress, the neurons showed no reduction from NBT to formazan. Striking NBT coloration of the somata, however, started to appear 30 min after reoxygenation, and the percentage of formazan-positive neurons increased during up to 32 h of the experimental period. About 50% of the neurons stained positively 24 h after hypoxia. This shows that rSOD is needed immediately after reoxygenation. Secondly, we examined whether rSOD exerts a cytoprotective activity against hypoxia. rSOD, with the application of proper treatment which incorporates rSOD into the target neuron, prevents the neuronal damage induced by oxide anions, which plays an important role in acute neuronal death accompanied by hypoxia. It can be concluded that, when intracellularly present in sufficient amounts, rSOD is useful for the treatment of hypoxic damage.