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Bazedoxifene (BZA), a selective estrogen receptor modulator (SERM), inhibits the action of estrogens on endometrial proliferation. Here, we evaluate the effect of a tissue-selective estrogen complex (TSEC) containing BZA and conjugated estrogens (CE) on ectopic endometrial lesions in a mouse model of endometriosis. Experimental endometriosis was created in 60 female CD-1 mice. The mice were randomly divided into 10 groups that received varying doses of either BZA (1, 2, 3, or 5 mg/kg/day), BZA (1, 2, 3, or 5 mg/kg/day) in combination with CE (3 mg/kg/day), CE treatment alone (3 mg/kg/day), or vehicle control for 8 wk. Treatment with BZA alone or the TSEC containing BZA/CE led to a decrease in endometriotic lesion size compared to controls. The mean surface area of the untreated lesions was 19.6 mm(2). Treatment with BZA or BZA/CE resulted in reduced lesion size (to 8.8 and 7.8 mm(2), respectively). No significant difference was found in lesion size between the BZA and BZA/CE treatment groups or between different doses of either treatment. Ovarian cyst formation was not evident in the treated groups. Treatment with the TSEC containing higher BZA dosages (3 and 5 mg/kg/day) led to significantly lower levels of estrogen receptor (Esr1) mRNA expression compared to the control treatment. No differences were observed in expression of progesterone receptor (Pgr). Immunohistochemical analysis also demonstrated a decrease in ESR protein. The combination of CE and BZA may prove to be a novel treatment option for endometriosis.