The present study was undertaken to determine the effect of estrogen and progesterone on the concentration of estradiol receptor in uterine tissue at the early pregnant rats. The result obtained are as follows : 1) The uterus wet weights of ovariectomized rats were decreased, while the concentration of estradiol receptor(E₂R) was increased, it seems to be well correlated with the low level of estrogen. 2) In the rats treated with estradiol, at 24 hours after treatment, the concentration of E₂R was higher than control-vehicle treatment-groups. 3) In the rats treated with estrogen and progesterone, the concentration of E₂R decreased, whereas estrogen primed rats treated with progesterone, the concentration of E₂R increased relatively. It is, therefore, concluded that the concentration of cytosol E₂R seems to be correlated with the peripheral estrogen and progesterone levels and then cytosol E₂R transfers into nuclear estradiol receptor to stimulate mRNA transcroption for the development of uterine tissue.