When cortical slices prepared from rat kidneys made ischemic were incubated under a 100% oxygen atmosphere, lipid peroxidation increased and the ATP level decreased. Such oxygenation of the slices was accompanied by decreases in gluconeogenesis and the glutathione level, but an antioxidant, N,N'-diphenyl-p-phenylenediamine, prevented the increase in lipid peroxidation without affecting decreases in ATP and glutathione levels, and gluconeogenesis. The results suggest that postischemic oxygenation of slices generates free radicals that cause the production of lipid peroxidation not associated with tissue injury.