Introduction: To investigate the effects of blocking prenatal androgen with the anti-androgen flutamide on testicular weight (TW), seminiferous tubular diameter (STD), testicular biopsy score (TBS), testicular testosterone ( TT), and fertility. Materials and Methods: Ten pregnant Wistar albino rats were injected with flutamide (100 mg/kg) on gestational days 16 and 19. Rats injected with the solvent were used as controls. Male pups from these dams were evaluated for testicular descent on postnatal day 22. The ability of each male to produce offspring was tested when the animals were aged 130 days. The rats were killed when 180 days old, and the testes were removed. Results: Sixteen of the 24 male rats (66.6%) exhibited cryptorchidism ( 6 unilateral, 10 bilateral) in the flutamide-treated group. Three of the rats with normally descended testicles were fertile, but none of the cryptorchid rats was fertile in flutamide-treated group. The average TW, mean STD, TBS and TT levels of the flutamide-treated rats was significantly lower than in the solvent-injected rats. However, there was no significant difference in these values between descended and undescended testes in flutamide-treated rats. Conclusion: Blocking of prenatal androgen with flutamide interferes with testicular development by inhibiting testicular descent, and also effects testicular morphology and function in both the descended and undescended testes of rats. Copyright (c) 2006 S. Karger AG, Basel.