The interaction of estradiol with rat uterus involves a two-step mechanism. Estradiol first binds to an extranuclear "uptake" receptor, forming a 9.5S complex. This process is temperature-dependent and does not require chemical modification of the steroid. The 9.5S complex then transfers estradiol to the nucleus, consuming the 9.5S receptor in the process. The formation of the 5S complex, which is involved in hormone retention and growth initiation, requires the presence of the 9.5S complex in the supernatant fraction. The 5S complex is formed from the 9.5S complex by a process that consumes 9.5S receptor and is slowed at low temperatures. The 9.5S receptor is extranuclear and is not saturated even at hyperphysiological estradiol levels, explaining the nonsaturable capacity of uterine tissue for initial estradiol uptake. The 9.5S complex may be transformed into the 5S complex by cleavage of the 9.5S receptor molecule. Experiments show that the 5S complex formation requires the presence of the supernatant fraction containing estradiol in the form of 9.5S complex. The study also demonstrates that the 9.5S receptor is heat-labile and is consumed during estradiol uptake. The findings suggest that the 9.5S receptor is involved in both estradiol uptake and the formation of the 5S complex. The study provides evidence that the 9.5S complex is extranuclear and that the 5S complex is formed from the 9.5S complex in the nucleus. The results indicate that the 9.5S receptor is not destroyed but is transformed into the 5S receptor by loss of a molecular fragment. The study also shows that the 9.5S complex can form spontaneously in the supernatant fraction, even at low temperatures. The 5S complex is not formed when estradiol is added to nuclear extract. The study was supported by grants from the National Cancer Institute and the American Cancer Society.The interaction of estradiol with rat uterus involves a two-step mechanism. Estradiol first binds to an extranuclear "uptake" receptor, forming a 9.5S complex. This process is temperature-dependent and does not require chemical modification of the steroid. The 9.5S complex then transfers estradiol to the nucleus, consuming the 9.5S receptor in the process. The formation of the 5S complex, which is involved in hormone retention and growth initiation, requires the presence of the 9.5S complex in the supernatant fraction. The 5S complex is formed from the 9.5S complex by a process that consumes 9.5S receptor and is slowed at low temperatures. The 9.5S receptor is extranuclear and is not saturated even at hyperphysiological estradiol levels, explaining the nonsaturable capacity of uterine tissue for initial estradiol uptake. The 9.5S complex may be transformed into the 5S complex by cleavage of the 9.5S receptor molecule. Experiments show that the 5S complex formation requires the presence of the supernatant fraction containing estradiol in the form of 9.5S complex. The study also demonstrates that the 9.5S receptor is heat-labile and is consumed during estradiol uptake. The findings suggest that the 9.5S receptor is involved in both estradiol uptake and the formation of the 5S complex. The study provides evidence that the 9.5S complex is extranuclear and that the 5S complex is formed from the 9.5S complex in the nucleus. The results indicate that the 9.5S receptor is not destroyed but is transformed into the 5S receptor by loss of a molecular fragment. The study also shows that the 9.5S complex can form spontaneously in the supernatant fraction, even at low temperatures. The 5S complex is not formed when estradiol is added to nuclear extract. The study was supported by grants from the National Cancer Institute and the American Cancer Society.