Inhibition of lipid peroxidation by nitroxide radicals in rat liver microsomes was studied using several nitroxide compounds which have various lipophilicities. Addition of NADPH to microsomes in an oxygen atmosphere induced lipid peroxidation as indicated by the increase of thiobarbituric acid (TBA)-reactive substances, hydroperoxide, conjugated diene, and oxygen consumption. Lipid peroxidation was inhibited by nitroxides located in both water phase and membrane. Oxygen consumption and the generation of active oxygens were inhibited by water-soluble nitroxides but not by intramembranous ones. Intramembranous nitroxides significantly prevented the production of conjugated dienes and lipid hydroperoxides. The facts suggest that water-soluble nitroxides interfere with the generation of active oxygens as preventive antioxidants, while intramembranous nitroxides inhibit the formation of lipid alkyl radical as chain-breaking antioxidants. The intensity of ESR signals due to intramembranous nitroxide radicals remained constant during the inhibition reaction of lipid peroxidation, suggesting that the prevention of lipid peroxidation is coupled with the reversible redox reaction between nitroxides and the reduced nitroxides in microsomes. The reduced forms of nitroxides, which were prepared by the reduction of intramembranous nitroxides, also inhibited the generation of TBA-reactive substances as well as the parent nitroxide. These facts indicate that either intramembranous nitroxide radicals or their reduced forms, "hydroxylamines," may be useful lipophilic antioxidants.