The degree of mitochondrial ADP/Fe/NADPH-induced lipid peroxidation was increased up to the fourth day after 9.0 Gy whole body gamma-irradiation. The lipid peroxidation inhibiting effect of succinate added to isolated mitochondria was diminished as a consequence of irradiation. The succinate, administered in vivo prior to irradiation, decreased the amount of malondialdehyde production and protected the succinate dehydrogenase enzyme against inactivation. The mean survival of succinate-pretreated animals was much longer than that of controls. The role of mitochondrial lipid peroxidation in the pathogenesis of radiation injury is discussed.