In women with hormone-dependent breast cancer, tamoxifen (TAM) frequently induces tumor regression, but regrowth occurs with continuation of antiestrogen therapy. Studies of breast xenografts in nude mice suggest that this secondary resistance to TAM may reflect the development of enhanced sensitivity to the estrogenic properties of TAM. In the current study, we examined the hypothesis that TAM could also induce a state of hypersensitivity to estradiol (E(2)) itself. Oophorectomized nude mice with MCF-7 cell xenografts received 25-mg implants of TAM [long-term TAM treated (LTTT) mice] or cholesterol (C-MCF-7) over a 5-month period (phase 1). The LTTT group regressed to a lesser extent than did C-MCF-7 tumors. After 4 months of TAM exposure, the LTTT tumors begin to regrow, as did the C-MCF-7, as assessed by slope analysis. At 5 months, TAM or vehicle implants were removed, and the LTTT and C-MCF-7 subgroups were given vehicle or two doses of E(2) to test estrogen sensitivity (phase 2). We used our "E(2) clamp" technique to maintain levels of plasma E(2) at either 1.25 or 20 pg/ml. Neither group responded to the very low concentrations of E(2) (1.25 pg/ml) or vehicle. The LTTT tumors but not C-MCF-7 tumors exhibited a growth response on exposure to 20 pg/ml E(2) during 7 weeks, as demonstrated with mixed models analysis. These studies provide evidence that long-term TAM exposure enhances sensitivity to the estrogenic effects of TAM and also to E(2) itself.