The purpose of the present study is to measure concentration of plasma estradiol and progesterone and to determine the concentration of the estradiol receptor in the uterine endometrium at the early pregnant rat.

The results obtained were as followings:

1) Plasma estradiol concentration showed the highest value on day 1 and the lowest on day 2, increasing again on day 3 and 4.

2) Concentration of plasma progesterone increased sharply on day 3, maintaining gradual increase thereafter. Rats ovariectomized on day 2, however maintained lower progesterone levels than those of day 1, throughout the preimplantational period.

3) The concentration of nuclear estradiol receptor seemed to be well correlated with the concentration of plasma estradiol throughout the preimplantation peried. Treatment of estradiol appeared to increase the concentration of estradiol receptor on day 3 and 6 in ovariectamized rats.

4) The differentiation of the uterine endometrial tissue seemed to be closely related to ovarian steroid hormones.