Aim: To study the effect of testosterone undecanoate (TU) injection on spermatogenesis in rats. Methods:Twenty adult SD rats received vehicle or TU (8 mg/kg, 19 mg/kg or 625 mg/kg) injection, im, every 15 days for 60days, and another 38 animals received similar treatments for 130 days with half of them undergoing a recovery phase of120 days (5 rats for each treatment). At the end of the treatment, testes were removed and the diameter of the seminif-erous tubules and the number of late elongated spermatids (steps 15-19) per testis were estimated with stereologicalmethods as a measure of the spermatogenic efficiency. Results: Low dose (8 mg/kg) TU treatment virtually hadno effect on spermatogenesis. A dose of 19 mg/kg slightly suppressed spermatogenesis 60 days after treatment, and se-vere suppression occurred after another 70 days of dosing. Spermatogenesis was completely recovered at the end of therecovery phase. Large dose (625 mg/kg) TU treatment did not significantly affect spermatogenesis and was we Methods: Twenty adult SD rats received vehicle or TU (8 mg / kg, 19 mg / kg or 625 mg / kg) injection, im, every 15 days for 60days, and another 38 animals received similar similar for 130 days with half of them undergoing a recovery phase of 120 days (5 rats for each treatment). At the end of the treatment, testes were removed and the diameter of the seminif-erous tubules and the number of late elongated spermatids (steps 15-19) per testis were estimated with stereological methods as a measure of the spermatogenic efficiency. Results: Low dose (8 mg / kg) TU treatment was hadno effect on spermatogenesis. A dose of 19 Spermatogenesis was completely recovered at the end of therecovery phase. Large dose (625 mg / kg) TU treatment did not significantly affect spermato genesis and was we