A proposed membrane-mediated mechanism of rapid non genomic response to estrogen has been the intense focus of recent research. Estren, a synthetic steroid, is reported to act selectively through a rapid membrane-mediated pathway, rather than through the classical nuclear estrogen receptor (ER)-mediated pathway, to maintain bone density in ovaierectomized mice without uterotropic effects. To further evaluate the mechanism and physiological effects of estren we studied responses in adult ovariectomized mice. In a three-day uterine bioassay, we found that 300 µg estren significantly increased uterine weight; in comparison, a more maximal response was seen with 1 µg estradiol (E2). The estren response was partly ER-independent, as ER knockout (ERKO) uteri also exhibited a more moderate weight increase. Estren induced epithelial cell proliferation in WT but not in the ERKO, indicating ER-dependence of the epithelial growth response. Examination of estren-regulated uterine genes by microarray indicated that early (2 h) changes in gene expression are similar to the early responses to E2. These gene responses are ER dependent, as they are not seen in the ERKO. Later estren-induced changes in gene expression (24 h) are blunted in comparison to those seen 24 h following E2. In contrast to early genes, these later estren responses are independent of ER, as the ERKO shows a similar response to estren at 24 h. We found that E2 or estren treatments lead to depletion of ER in the uterine cytosol fraction and accumulation in the nuclear fraction within 30-60 min, consistent with an ability of estren to regulate genes through a nuclear ER rather than a non-genomic mechanism. Interestingly, estren, but not E2, induces accumulation of androgen receptor (AR) in the nuclear fraction of both WT and ERKO samples, suggesting that AR might be involved in the later ER-independent genomic responses to estren. In conclusion, our studies suggest that estren is weakly estrogenic in the mouse uterus and might induce nuclear ER and AR mediated responses. Given its activity in our uterine model, use of estren as a bone selective clinical compound needs to be reconsidered.