Abstract: Objective To investigate the adverse effect and its mechanism of β-cypermethrin( β-CP)on gonads in female immature mice.Methods Fourty specific pathogen-free immature mice were randomly divided into a negative group(treated with earhnut oil)and three β-CP exposure groups(0.1,1.0,10 mg/kg body weight).A daily subcutaneous exposure was conducted for 28 days.The time of vaginal open,estrous cycle,estrogen in serum,gonads(ovary and uterus)wet weight and coefficient,and the expression of estrogen receptor(ER)in endometrial were measured.Results Compared with the control group,the time of estrus shortened and the time of diestrus extended.The wet weight of uterus(0.109±0.011g) increased in the high dose group(P<0.05),but the coefficient and the blotted weight of uterus,the level of estrogen in serum and the sum of gonadal in endometrial were not significantly different among the groups(P>0.05 for all).In 1.0 and 10 mg/kg dose groups,the ratios of ER expression in endometrial was 56.00±3.32% and 29.00±5.58% and were lower than those of in the control group(P<0.05).Conclusion β-CP has gonadal subchronic toxicity and endocrine disrupting effect and can induce the expression of ER in uterus in female mice.