To evaluate the effect of soy isoflavone extract (SIE) on the uterus and urethra of castrated female rats. Ovariectomized adult virgin Wistar rats (n = 45) received, for 30 days: placebo (GI); SIE from day 5 of castration (GII); or day 28 of castration (GIII). A standardized SIE was administered by gavage at 125 μg genistein/g body weight/day, with a soy-free diet. Average numbers of nuclei (NU), blood vessels (BV) and collagen fibers (CF) in uterus and mid-urethra sections were compared using Dunnett and Tukey tests (95% CI). Uterus: GII and GIII differed from GI in all respects; GII vs. GIII, no differences. Urethra: GIII vs. GI: no differences; GII vs. GI, differences in NU and CF; GII vs. GIII, differences in NU and CF. Early oral administration of SIE to ovariectomized rats reversed uterine/urethral effects of castration.