The effects of estradiol (E), progesterone (P), cortisol (C), and combinations of the three hormones on the uterus luminal epithelia, mitotic index, and leukocyte infiltration were studied. At 24 hours, E treated groups showed increased uterine weight. The highest weight increase was observed in groups treated with E alone, and the maximal atrophic picture in terms of the luminal epithelia was found in the group treated with C + P. At 48 hours, only E and C + E showed increased uterine weight. At 24 hours, all groups treated with E showed increased mitotic figures. The highest values were found in groups treated with E, and with C + E + P. At 24 hours, C and P inhibited the E effect. At 48 hours, only E and C + E treated groups showed a mitotic increase. It is proposed that C antagonizes P or potentiates some E actions.