The present investigation has been undertaken to understand the mechanism of implantation process, by demonstrating the role of ovarian steroids in the differentiation of uterine endometrium for implantation. We observed that the phosphatase activities seem to be dependent upon the ovarian steroids ; that is the activity showed higher level in progesterone treated group than in estradiol treated one, and the highest activity was found in the group treated with both estradiol and progesterone. The differentiation of the uterine endometrial tissue seemed to be closely related to ovarian steroid hormones ; estradiol seemed to effect on the differentiation of the luminal and glandular cell and progesterone on the formation of collagen fiber of the stromal cells, thereby inducing decidualization. Ovarian steroids might stimulate phosphatase synthesis thereby inducing differentiation of luminal and glandular tissues and then development of the uterine stromal cell for triggering decidualization in pregnant rats.