The present study was undertaken to assess in vitro the endometrial extraction of natural estrogens. Normal, hyperplastic, and neoplastic endometria were studied. This was accomplished by the use of double isotope, single-injection techniques performed during the extracorporeal perfusion of human isolated uterus. The differential permeability of vascular beds in normal and neoplastic endometrium to estrogens was evaluated. The effects of binding by human serum proteins on estrogen influx into the endometrium were also determined. When protein-free Ringer's solution was used as an injection vehicle, both normal and abnormal endometrium permitted free diffusion of estradiol (E2), estrone (E1), and estrone sulfate (E1S). In contrast, the endometrial extraction of these estrogens from human female serum was significantly lower than that obtained with Ringer's solution. The extraction of E2, E1, and E1S from human serum was significantly higher in hyperplastic and carcinomatous endometria than in normal proliferative endometria. We conclude that 1) membrane permeability to estrogenic influxes differs between normal and abnormal endometria and 2) plasma proteins decrease the endometrial uptake of estrogens.