SUMMARY Priming of ovariectomized mice with 0·1 μg. oestradiol-17β for 3 days modified the subsequent cellular response of the uterus to oestrogen and progesterone. A single dose of oestradiol-17β (0·02 μg.), or progesterone (1 mg.), or of both hormones given simultaneously, stimulated mitosis in the epithelial cells of the endometrium (although not in the stromal cells). However, the same hormones administered to primed mice stimulated considerable stromal cell division and reduced epithelial mitosis. The maximal effect was found when both hormones were given on the 4th day after the end of priming. It is suggested that the priming of the uterus by oestrogen secreted by the ovary before mating is concerned in the abrupt change from epithelial to stromal cell division that occurs between days 3 and 4 of pregnancy, and is of importance in the sensitization of the uterus for implantation.