Abstract: Objective To explore the effects of prenatal soyisoflavone exposure on reproductive system of offspring male rats.Methods 40 preg nant rats which mated with male rats according to 2:1 ratio,were divided into four groups randomly.Soy isoflavone was conducted to the pregnant rats by garage(peanut oil.worked as solution,was delivered to control group) during the period of E13~E19.F1 male rats were sacrificed on 90^th day.Results The measured indices of F1 male rats in the 150 and 450mg/kg soy isoflavone exposure groups were:aver age testis weights 2.68 and 2.16g,alkaline phosphatase activities 1078.88 and 1058.88 U,levels of lactate dehydro genase 2 678.88 N and 529.38 U,sperm motility 20.12% and 24.13%,sperm counts in the tail of epididymis 50×10⁶/g and 40.5×10⁶/g,sperm productions per day 19.4610⁶(g·d)and 16.2110⁶/(g·d)respectively.All indices mentioned decreased significantly compared with those in the control group and showed does-effect relationship with soy isoflavone.Conclusion High dose of soyisoflavone exposure might bring hazards to the F1 male reproductive system in differentiation period of reproductive system.