The present investigation has been undertaken to understand the mechanism of implantation process, by demonstrating the role of ovarian steroids in connection with phosphatase activity in the differentiation of uterine endometrium for implantation. The results obtained are as followings: The differentiation of the uterine endometrial tissue was closely influenced by the ovarian steroid hormones; at first, 17${eta}$-estradiol initiated the differentiation of the uterine luminal and glandular epithelial cells, and then progesterone induced differentiation of stromal cells, and thereby two steroids maintain decidualization of the uterine tissues. 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. Acid phosphatase showed the highest activity whereas alkaline phosphatase showed the lowest in the rat uterine endometrium during early pregnancy.