It has been reported that Ginsenoside can increase body resistance to many harmful factors and protect tissues from damage when an organism is in stress. To understand the mechanism of this action, a study on the antioxidative effect of Ginsenoside Rb1 and Rg1 was carried out. Results showed that Ginsenoside Rb1 and Rg1 could inhibit lipid peroxidation of rat liver and brain microsomes and that Rb1, at the final concentration of 10(-4)-10(-3) mol/L, could scavenge O2-. induced by liver microsome-NADPH-gossypol system. In in vivo experiment, Rb1, at a dose of 50 and 25 mg/kg/day x 3 ip, inhibited MDA formation in liver homogenate of rats by 26.8% (P less than 0.05) and increased the activities of catalase and GSH peroxidase by 47.2% (P less than 0.001) and 96.4% (P less than 0.001), respectively. However, no change in the activity of superoxide dismutase was found in liver cytosol of rats treated with Rb1.