The site of sperm capacitation, the agents and mechanisms causing capacitation and acrosome reaction (AR) in vivo are not well understood. The female reproductive tract has been reported to playa key role during capacitation and AR. Some experiments were carried o. ut on the capacitation and AR of hamster epididymal spermatozoa in the estrogen and progesterone dominated uterus (estrous and diestrous respectively) albino mice, incubated in TALP without calcium and BSA. Also the effect of estradiol (200! lg/ml) supplemented to TALP, on capacitation and AR was examined. Capacitation and AR of hamster spermatozoa incubated in the isolated uterus of both estrous and diestrous mice were significantly (P< 0.05) higher in the presence of exogenous estradiol than that in its absence. Acrosome shedding occurred earlier ie at 3rd hour as compared to the in vitro studies where it occurred at 5th hour. The present study thus reveals that uterus of both estrogen and progesterone dominated mice play an important role in the induction of capacitation and AR. The addition of estradiol might have the influence on the synthesis of uterine proteins of mice which might be important for capacitation and AR.