Abstract Estrone and estradiol have been measured in subcellular fractions of endometria from women taking estrogen either alone or in combination with norethisterone. Within the nucleus there was a four-fold excess of estradiol over estrone in endometria from women taking estrogen only. This ratio was diminished to near-unity when norethisterone was also taken. The diminished nuclear estradiol content in the norethisterone treated endometria correlated with lower nuclear estradiol receptor content and increased estradiol dehydrogenase activity. In all cases, there was an excess of estrone over estradiol in the plasma. It is suggested that in these postmenopausal women, estradiol is the most active intranuclear form of estrogen and that progestins such as norethisterone can diminish the nuclear estradiol content via a mechanism involving estradiol dehydrogenase. Some characteristics of the nuclear estradiol receptor from these estrogen-primed endometria have been studied. As judged from labelling with [ 3H]-estradiol at 4° and 30°C, the nuclear receptor behaves as if it were uncomplexed with estradiol. Highly purified nuclei behave in the same manner.