The quantitative analyses of the phosphatase activity in the endometrium of the rat ovariectomized on Day 2 of pregnancy was carried out in comparison with the intact one, in order to investigate the hormonal dependency of the uterus prior to the implantation, and to study the phosphatase activity in the endometrial tissues in vitro incubated in different acidity of the medium. The results obtained were as follows: 1. The activity of the total phosphatase was the highest at Day 3 of pregnancy of the intact animals irrespective of acidity of the medium. However, the ovariectomized rat showed its peak somewhat delayed. The time of the highest activity of the enzymes was matched with the time of high secretion of the ovarian hormones. 2. The activity of acid phosphatase in the endometrium was twice or four times as much high as that of neutral or alkaline phosphatase, respectively. 3. The activity of alkaline phosphatase was rather steady in Day 3 through Day 5 of the pregnancy of the rat intact or ovariectomized but with low level compared to those of other phosphatase. 4. The present re~lt indicated more important role by $Mg^{2+}$-dependent phosphatase than by $K^+$-dependent one for the preparation for decidualization.