PIP: This report describes experiments designed to answer several important questions about the biochemistry of estrogen-stimulated postmenopausal endometrium; in particular; how much estrogen enters the endometrium and the biological effectiveness of that estrogen in women receiving different forms of postmenopausal estrogenic therapy. To this end, nuclear and cytoplasmic estradiol receptor (ER) and cytoplasmic progesterone receptor (PR) were measured in curettage samples of endometria from women receiving, either sequentially or cyclically, Premarin, Harmogen, Progynova, or mestranol at either low or high doses. Cyclical treatment with estrogen alone was compared with sequential therapy with 3 weeks of estrogen plus 1 week of estrogen plus 1 week of estrogen plus northisterone. No difference in any of the receptor levels was found in samples obtained during the 3rd week of any of the 4-week treatment cycles. For 2-3 weeks of a treatment cycle, the receptor levels were similar to those seen in premenopausal, proliferative-phase endometrium, suggesting that postmenopausal endometrial cells are subjected to a very potent estrogenic stimulus for a considerable period. Norethisterone ingestion for 1 week decreased both the amount of nuclear ER and the percentage of total cellular ER that were in the nuclear fraction. Estradiol dehydrogenase was also induced by the progestin. The presence of this enzyme could result in lowered nuclear ER levels which were seen during the progestogenic phase of the treatment schedule. Nuclear ER was lower during Week 3 than Week 2 of estrogen treatment. In addition, PR was negatively correlated with nuclear ER in postmenopausal tissues obtained in Week 3 in contrast to positive correlations seen in premenopausal samples. Possibly a 3-week treatment with estrogen leads to a refractory condition. When receptor levels of normal, cystic hyperplastic, typical hyperplastic, and atypical hyperplastic tissues were compared, the endometria that had been returned to normal histology suggested that cells in atypical hyperplastic endometrial cells may be more estrogen sensitive than other types of endometrial tissues.