This study was undertaken to examine the effect of Jwagyuyeum and Woogyuycum, being known to reinforce Kidney-yin and yang, on the activities of endogenous antioxidant enzymes and the production of oxygen free radicals in the liver and kidney tissues, Alterations in enzyme activities were observed after in vivo treatment in rats, Jwagyuyeum and Woogyuyeum caused a significant increase in the activities of superoxide dismutase(SOD) and catajase Jwagyuyeum significantly increased the activity of glutathione peroxidase in both liver and kidney, but the enzyme activity was not significantly altered by Woogyuyeum. Treatment in vitro of Jwagyuyeum and Woogyuyeum decreased the production of oxygen free radicals in a dose-dependent fashion. These results suggest that Jwagyuyeum and Woogyuyeum stimulate the activities of antioxidant enzymes and inhibit directly the production of oxygen free radicals. These effects of both herbs may contribute to prevent the oxygen free radical-induced impairment of cell function.