Objective: To evaluate Ki-67 antigen expression in the mammary epithelium of female rats in persistent estrus treated with raloxifene. Materials and Methods: Forty-one Wistar-Hannover rats in persistent estrus induced by1.25 mg of testosterone propionate were randomly divided into two groups: Group A (control, n=21) in which the animals received only the vehicle (propylene glycol) and Group B (experimental, n=20) in which the rats received 750 μg/day of raloxifene by gavage. After 21 days of treatment, all the animals were sacrificed and the first pair of abdominal-inguinal mammary glands was extirpated and fixed in 10% buffered formalin to investigate Ki-67 expression by immunohistochemistry. The data were analysed using Student’s t-test (p < 0.05). Results: The percentage of Ki- 67-stained nuclei per 500 cells in the mammary epithelium was 42.33±6.18 and 15.51±3.71 [mean ± standard error of the mean (SEM)] in the control and experimental groups, respectively (p < 0.001). Conclusion: Raloxifene treatment significantly reduced Ki-67 expression in the mammary epithelium of rats in persistent estrus.