Breast tumors from postmenopausal women contain levels of estradiol similar to those in premenopausal patients even though serum estradiol levels fall by an order of magnitude upon cessation of ovarian function. The present study sought to examine enhanced uptake from plasma as one potential mechanism for maintenance of high tissue estradiol levels in postmenopausal patients. Accordingly, we used osmotic minipumps to continuously infuse estradiol (E2) at rates producing serum concentrations ranging from pre- to postmenopausal levels for two weeks to oophorectomized Sprague-Dawley rats bearing nitrosomethylurea-induced mammary tumors. We then measured E2 concentrations in various tissues and sera and reasoned that tissue affinities for estradiol could be directly calculated from in vivo measurements by adapting Scatchard analysis to steroid infusion data. Using this method, we demonstrated a very high affinity estradiol binding component with a Kd two orders of magnitude higher (i.e., 0.35 x 10(-12) M) than determined with standard in vitro techniques. A second estradiol binding component with the expected Kd of 1 x 10(-10) M was also present. Estradiol bound to both classes of binding sites could be 98% displaced with diethylstilbestrol within a 6-hr period. In vivo steroid binding off-times calculated from log-linear slopes averaged approximately 60 min. These data demonstrated that the actual E2 binding affinity in target tissues in vivo, especially at low estrogen concentrations, is much higher than usually estimated from standard, in vitro estrogen receptor assays. These observations provide one mechanism to explain why estradiol concentrations remain high in breast cancer tissue from postmenopausal women and consequently can stimulate tumor proliferation.