BACKGROUND Rhodiola crenulata is a Tibetan mountainous plant, commonly used in Eastern alternative medicine. Many phytochemicals possess estrogenic activity, a critical regulator of proliferation in mammary epithelial cells. We have previously characterized anti-cancer properties of R. crenulata in aggressive triple negative breast cancer cells, lacking the expression of estrogen receptor. Currently, it is unknown whether R. crenulata exerts estrogenic effects and as such consumption may be a concern for women with estrogen receptor positive breast cancer that use Rhodiola sp. to relieve mild to moderate depression. PURPOSE In this study, we wished to determine whether a hydroalcoholic fraction of the R. crenulata root extract exhibits estrogenic activity in estrogen receptor positive (ER+) breast cancer cells in vitro and whether it affects normal mammary epithelial ER target gene expression in vivo. METHODS ER transcriptional activity was analyzed in MCF7 cells expressing an ERE reporter construct and confirmed via qPCR of endogenous ER target genes. We also monitored cellular proliferation over time. Additionally, to assess stem-like properties in MCF7 cells, we performed a tumorsphere formation assay under anchorage independent conditions. We examined whether R. crenulata treatment reduced $$-catenin levels via Western blotting and measured $$-catenin transcriptional activity by a reporter assay. To examine the effects of R. crenulata on normal mammary epithelial cells, we performed immunohistochemical staining of ER and PR in the mammary glands of mice fed R. crenulata for 12 weeks. RESULTS We show an initial activation of ER transcriptional activity by dual reporter assay, qPCR and proliferation of MCF7 ER+ cells in response to 24 h of R. crenulata treatment. However, upon longer treatment basal and R. crenulata induced transcriptional activity was suppressed. There was a decrease in cell doubling times and a decrease in tumorsphere formation. In association with these changes, ER$$ transcript levels were decreased and active $$-catenin levels were reduced in the cells treated for 2 weeks. Finally, we show no change in estrogen targets in normal mammary cells in vivo. CONCLUSION These data suggest that the R. crenulata extract contains components with estrogenic activity. However, R. crenulata treatment could still be protective in ER+ breast cancer cells, as longer treatment reduced the transcriptional activity of $$-catenin and ER responses leading to reduced proliferation and tumorsphere formation. Furthermore, administration of 20 mg/kg/day R. crenulata to mice did not have an observable effect on mammary epithelial ER$$ target gene expression in vivo.