Wet weight of the vagina increased by more than two-fold following treatment of the ovariectomized rabbits daily for 7 days with 100 µg/kg estradiol- 17Β or estriol. The concentration of estrogen receptors expressed on the basis of milligrams of protein in both the cytosolic and nuclear fractions decreased significantly after treatment with estradiol or estriol. The data reinforce the view that estriol given in low doses over a long period of time has as good an effect as estradiol on the vagina, and its use might be safer with respect to the effect on endometrial proliferation.