Abstract: Daidzein 3 mg/(100 g body weight·day) was injected subcutaneously for 16 days in male,female and castrated male rats.To the male rats,average daily body weight gain increased by 14.7% (P<0.05),feed consumption to body weight gain ratio decreased by 12.0%,hind leg muscle weight,total RNA content of hind leg muscle increased significantly,and no apparent difference was found in total DNA content.Blood urea nitrogen decreased significantly,whereas serum testosterone,estradiol,β-endorphin and growth hormone increased obviously.To the castrated male rats,no considerable differences were found in average daily body weight gain and hind leg muscle weight,although concentrations of serum growth hormone,β-endophin and testosterone increased significantly.However,the testosterone level of castrated male rats was about 8.0% of that of intact male rats.To the female rats,the average daily body weight gain decreased by 19.0% (P<0.01).The levels of serum testosterone,estradiol and growth hormone significantly decreased.The results showed that the effects of daidzein on muscle growth and some endogenous hormone levels in rats exhibited sex difference.The promoting effect of daidzein on male rats seems depending on the function of testis.