The effects of antiestrogens, tamoxifen and ICI 182,780, and aromatase inhibitors, arimidex (anastrozole ZD1033) and letrozole (CGS 20,267), on the growth of tumors were studied in nude mice. In this model, estrogen dependent MCF-7 human breast cancer cells stably transfected with the aromatase gene were inoculated in four sites per mouse. Sufficient estrogen is produced from aromatization of androstenedione supplement (0.1 mg/mouse/day) by the cells to stimulate their proliferation, tumor formation, and maintain the uterus similar to that of the intact mouse. Once the tumors reached a measurable size, the mice were injected with antiestrogen or inhibitor for 35-56 days. Tumor volumes were measured weekly. At autopsy, the tumors were removed, cleaned, and weighed. Statistical data was determined from tumor weights. Both antiestrogens were effective in reducing tumor growth in these mice. Tamoxifen appears to be more effective than ICI 182,780, although the former stimulated the uterine weight whereas the pure antiestrogen did not. However, both aromatase inhibitors were more effective than the antiestrogens. Tumor regression was observed with letrozole. Thus, after-treatment tumor weights were less than those of a group of mice at the start of treatment. The aromatase inhibitors also reduced the weight of the uterus, suggesting that these compounds, as well as the pure antiestrogen, may not cause endometrial proliferation, unlike tamoxifen. These aromatase inhibitors may not only benefit patients who have relapsed from tamoxifen, but may be more effective in patients as first line agents for suppressing the effects of estrogen.