OBJECTIVE To investigate the effects of soybean isoflavones （SI） on the reproductive development of young mice. METHODS C57BL/6 young mice were randomly divided into control group， SI low-dose and high-dose groups （10， 100 mg/kg）， with 10 mice in each group （half male and half female）. The young mice in each group were given corresponding liquid intragastrically， once a day， for 2 consecutive weeks. After the last administration， the percentage of body weight increase was calculated； serum estradiol and testosterone levels， malondialdehyde （MDA） content， total antioxidant capacity （T-AOC）， superoxide dismutase （SOD） and glutathione peroxidase （GSH-Px） activities in the reproductive organs of the young mice were determined. The histopathological changes in the reproductive organs were observed. The cell apoptosis of reproductive organs was detected. RESULTS Compared with the control group， the percentage of body weight increase in female mice was increased significantly in the SI high-dose group， while that of male mice was decreased significantly （P＜0.05 or P＜0.01）. Cystic follicles could be seen in the ovarian tissue in SI groups， a loose arrangement of spermatocytes could be seen in the testicular tissue， and partial epithelial cell shedding could be seen in epididymal tissue. The serum level of testosterone in female young mice and the serum levels of testosterone and estradiol in male young mice in SI groups， GSH-Px activity in the ovarian tissue of female young mice in the SI low-dose group， T-AOC activities in the ovarian tissue of female young mice in SI groups as well as the apoptotic rates of cells in testicular and epididymal tissue of male young mice in SI groups were increased significantly （P＜0.05 or P＜ 0.01）； the serum level of estradiol in female young mice in SI groups， SOD activity in the ovarian tissue of female young mice in the SI high-dose group， and MDA contents in the ovarian tissue of female young mice in SI groups as well as the apoptotic rates of cells in ovarian tissue of female mice in SI groups were decreased significantly （P＜0.05 or P＜0.01）. CONCLUSIONS SI can enhance the antioxidant stress capacity of ovarian tissue in female young mice and reduce their oxidative stress damage， but it has certain toxicity to reproductive organs in male mice.