The effect of saponins isolated from the stem, leaf and rhizoma of panax ginseng on the formation of lipid peroxides in aged (30, 24 and 20 months) and middle-aged (14 months) wistar rats was studied with both in vivo and in vitro methods. The results showed: The formation of lipid peroxides increased with age in rats and mice: the two saponins of ginseng stem, leaf and rhizoma can obviously inhibit the lipid peroxidation in the serum (plasma) and liver homogenate of aged and middle-aged rats (p less than 0.05 and less than 0.001). The same effect was also shown in the aged LACA mouse (18 months) experiment.