This paper contains: A review of the features implicated in estrogen action on uterine cells: metabolism, transport, entry, cytosol receptor, and first effects. (2) New results concerning the interaction of several estrogens and antiestrogens with the receptor. Binding of radioactive estrogens: at equilibrium at 0 °C, KD (in nM) of estradiol 0.2–0.5, estrone 2.0, and estriol 1.3. Inhibition constant at equilibrium measured by competition with nonradioactive compounds of natural and synthetic steroid and nonsteroidal estrogens, new stilbene derivatives, phytoestrogens, etc. (28 compounds). Kinetic studies. Same dissociation rate constant for estradiol, estrone and estriol and association rate constant greater for estradiol. Formation of the neonuclear steroid receptor complex by estrone and estriol, implicating that estradiol is not unique for promoting the cytosol receptor translocation to the nucleus. (3) A review of possible mechanisms for antiestrogen action.