A single dose of 6-methyluracil (50 mg/kg) administered intraperitoneally to BALB and SHK mice prior to X irradiation (5.0 Gy) produced a pronounced radioprotective effect which was dependent upon the initial antioxidant status of the body, but independent of the effect of the agent on postirradiation leukopenia dynamics. The radioprotective effect of 6-methyluracil was conditioned by its influence on lipid peroxidation regulation and was displayed by the normalization of the antioxidant activity of lipids of radiosensitive organs and tissues and by high activity of superoxide dismutase in the liver and erythrocytes.