Uterine leiomyomas (fibroids) are the most common gynecological neoplasms and may be associated with significant morbidity. Recently, we described a rat model (Eker rat) of fibroid development in which reproductive tract leiomyomas develop spontaneously with high frequency. The present studies describe the estrogen and antiestrogen responsiveness of an Eker rat leiomyoma-derived cell line in vitro and a nude mouse xenograft system in vivo. In this cell line, estradiol stimulated growth in estrogen-depleted medium, whereas the nonsteroidal antiestrogen tamoxifen maximally inhibited cell proliferation in medium containing 10% charcoal-stripped serum. Proliferation was also decreased by the biologically active tamoxifen metabolite 4-hydroxytamoxifen; the metabolite was more effective than the parent compound in exerting this growth inhibition. Compared to placebo-treated controls, estradiol increased the size of tumors that developed in a nude mouse xenograft system, whereas tamoxifen increased tumor latency and decreased tumor size. This study of leiomyoma cells in a well defined system suggests that antiestrogens may prove efficacious in the treatment of this clinically important neoplasm.