OBJECTIVE: 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. DESIGN: Controlled prospective study. SETTING: University research laboratory. ANIMAL(S): Thirty sexually mature female Wistar-Albino rats that were 20 weeks of age. INTERVENTION(S): 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. MAIN OUTCOME MEASURE(S): Number of mature follicles, endometrial thickness, and serum levels of hormones. RESULT(S): 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. CONCLUSION(S): In rats, the effect of letrozole on follicular maturation is similar to that of clomiphene citrate.