Background: Anabolic-androgenic steroids are used at high doses by athletes for improving athletic ability, physical appearance and muscle mass. Therefore, the abuse of these steroids has been significantly increased. Many undesirable side effects of these steroids on the male reproductive function have been reported, however, little is known about their effects on sexual behavior and tissues of the reproductive system. The aim of this study is to identify the effects of anabolic-androgenic steroids on the body, testis and epididymis weight, as well as semen parameters. Method: Five groups of Sprague-Dawley adult male rats (n=72) were used. Two experimental groups were medicated with intramuscular injection of 3 and 10 mg/kg body wt/wk of nandrolone decanoate and two vehicle groups with same doses of sweet almond and olive oils, respectively, for 14 weeks. The control group received no injection. One week after the last injection, rats were sacrificed and the weights of the body, testis and epididymis and also semen parameters were assessed. Results: The weights of testis and epididymis, as well as, sperm count and motility rate were significantly decreased in experimental groups than in the vehicle and control groups. Morphologically abnormal sperms were observed. Conclusion: We found that anabolic-androgenic steroids affect fertility parameters and cause testis atrophy.