Chronic dietary restiction without malnutrition extends both the mean and maximum life span of all laboratory organisms so far tested. This life-extension is attributed to numerous beneficial effects of well-maintained physiological systems, and the retardation of chronic diseases. Although the biological mechanisms of these anti-aging actions by dietary restriction are not known, recent evidence indicated that modulation of free radical activity might play important roles in such beneficial action. Our present study with SPF male Fischer 344 rats provides further data that the anti-radical action of dietary restriction suppresses the formation of reactive oxygen species, enhances the antioxidant capacity, and increases the membrane resistance against oxidative stress. Based on these results we propose that dietary restriction is an effective modulator of free radical activity.