When goat spermatozoa were preincubated for 4-6 h and 6 h in the uteri isolated from hamster and rat, and for 6 h in the hamster uterus in situ, they developed the ability to penetrate zona-free hamster eggs in vitro. Zona-free hamster eggs were not penetrated after insemination with goat spermatozoa preincubated in the isolated hamster uterus 4 h before and 2 h after expected time of ovulation, respectively. Zona-free hamster eggs were not penetrated after insemination with goat spermatozoa preincubated for 4 h in the isolated hamster uterus, but 10 and 18% of eggs were penetrated by spermatozoa preincubated for 5 and 6 h in the isolated uterus.