The present study was designed to investigate the effects of genistein, daidzein, and quercetin on the antioxidative systems of normal rats. Male Sprague-Dawley rats were divided randomly into seven groups and treated with flavonoids at either 2 or 20 mg/day or through vehicle for four weeks. Lipid peroxidation in the liver was inhibited significantly following administration of quercetin. Genistein and daidzein did not have significant effects except in rats treated with 20mg daidzein/day. Genistein and daidzein treatment did not affect the content of $alpha$-tocopherol in the serum and liver, while quercetin caused a slight increase. In hepatic glutathione and its related enzymes, genistein and daidzein treatment tended to cause a decrease in $alpha$-tocopherol content, although no significant difference was found. However, quercetin treatment significantly decreased the content of glutathione together with the activity of glutathione reductase in all doses in the liver but there was no significant difference in the brain. Interestingly, daidzein treatment in the brain at 2mg/day significantly increased glutathione (27.1% p＜0.05) compared with the control group, while at 20mg/day glutathione decreased significantly (26.6%, p＜0.05). In conclusion, genistein has not antioxidant effects. Daidzein quercetin may have the capacity to produce not only antioxidants but also have adverse effects including the production of pro-oxidants. Therefore, people should consider consumption at a high dosage.