The present article describes some of the factors regulating progesterone action in mammalian uterus. The main emphasis is laid on the role of cytosol receptor concentrations in the ultimate expression of progestin action. In addition, recent findings on the regulation of uteroglobin (a progesterone-stimulated rabbit uterine protein) are dealt with, especially in the light of estrogen-progestin interaction in this regulation. The current evidence suggests that estradiol is able to mimic and modify progesterone action in a dose-related fashion: small doses of estradiol potentiate the progesterone effect, whereas higher concentrations of this steroid almost completely block expression of progestational response. These actions of estradiol seem to occur via mechanisms distal to progesterone receptor synthesis, since the doses of estradiol which most markedly inhibit progesterone action on uteroglobin synthesis bring about a significant increase in uterine progesterone receptor concentrations.