To compare the effects of letrozole and clomiphene citrate in the rat in terms of number of ovarian follicles; endometrial thickness; and serum levels of E2, FSH, LH, and T. Controlled prospective study. University research laboratory. Thirty sexually mature female Wistar-Albino rats that were 20 weeks of age. Letrozole, 5 mg/kg of body weight daily (10 rats); clomiphene citrate, 100 microg/kg daily (10 rats); or saline solution, 2 mL/d (10 rats). After 2 days, rats were euthanized and ovariectomized. Number of mature follicles, endometrial thickness, and serum levels of hormones. Mean levels of FSH, LH, E2, and T; number of mature follicles; and ovary size differed among the groups, whereas the mean endometrial thickness did not differ. In rats, the effect of letrozole on follicular maturation is similar to that of clomiphene citrate.