Why some women are at increased risk for the development of endometrial carcinoma while taking the antiestrogen tamoxifen (Tam) for breast cancer treatment or prevention is unknown. Various strains of rodents display differences in sensitivity to compounds with estrogenic activity, but whether differences in Tam sensitivity exist in rodent strains has not been investigated. In the present study, we investigated whether rat strain differences in reproductive tract sensitivity to Tam and estrogen exist between Fischer 344 (F344) and Sprague Dawley (SD) rats. Immature (21–23 day;n=6/group), ovariectomized F344 and SD rats were treated with vehicle (control), 17&bgr;-estradiol (E2) [1×10−6to 1.0 μg/kg body weight (BW)] or 4-OH tamoxifen (4-OHT) (1×10−4to 10 mg/kg BW) for 2 days and then sacrificed on day 3. Reproductive tracts were collected, weighed, and examined for changes in histomorphology and expression of ER&agr;and nuclear receptor co-regulators (SRC1, p300, CARM1, GRIP1, SPA, REA and Uba3). Treatment with E2(1×10−5 μg/kg BW) increased (p<0.05) uterine epithelial cell height in F344 but not SD rats, demonstrating increased sensitivity of the F344 strain to E2. Conversely, treatment with 1×10−3 mg/kg BW 4-OHT increased (p<0.05) uterine weight and epithelial cell height in SD but not F344 rats, demonstrating that the SD strain is more sensitive to the antiestrogen. Northern and Western blot and immunohistochemical analysis revealed that ER&agr;expression levels in the SD and F344 uterus were not different. Expression of receptor co-regulators was higher in the uterus compared to the vagina regardless of strain and higher CARM1 expression was seen in SD uterus compared to F344 rats. Understanding differences in Tam sensitivity may help us to better understand why some women develop endometrial cancer while taking Tam and be beneficial in treatment decisions for breast cancer patients.