ObjectiveThe objective of the study was to compare the effect of tamoxifen and raloxifene on the endometrium of female rats in persistent estrus, by Ki-67 protein expression.MethodsThe study comprised 60 Wistar-Hannover female rats in persistent estrus, induced by a single subcutaneous dose of 1.25 mg of testosterone propionate on the second day of age. At 90 days of life, the animals were randomly divided into 3 groups of 20 animals each. Group 1 (control), received only placebo; group 2, the animals were treated with tamoxifen, 250 μg/d; and group 3, the rats were treated with 750 μg/d of raloxifene by gavage during 30 days. Then, the animals were killed, and the endometrium was removed for immunohistochemical analysis of Ki-67 antigen expression. Statistical analysis was performed by β regression model (P < 0.05).ResultsMean percentages of Ki-67 protein expression in the endometrium of rats in persistent estrus were 43.21% ± 3.39%, 7.36% ± 0.95%, and 7.20% ± 0.76% in groups 1, 2 and 3, respectively (P < 0.001). There was no statistical difference between groups 2 and 3 (P = 0.7159).ConclusionsThe present results indicate that, at the doses and during the time of treatment used, both tamoxifen and raloxifene induce atrophy in a similar way of endometrial epithelium of rats in persistent estrus.