Twenty adult male rats three month old, weighing 80-190 gm were obtained from the Animal House Assiut University and divided into one control group and three treated groups (5 rats each group). Treated groups were injected intramuscular with Nandrolene Decaonate at doses of 2.5,5, and 10mg/kg body weekly for three months. At the end of exp. the whole body weight and the seminal vesicle weight of both treated and control group were recorded. The relation wight of the seminal vesicle to the body weight was increased significantly in treated group compared to the control. The thickness of the smooth muscle fiber layer also was increased in treated groups compared to the control group. Histopathologically, shortening of the mucosal folds, abundant seminal secretions, fragmentation of the smooth muscle fiber layer and hemorrhage and hyperemia below the muscular layer were observed. Desquamation of the epithelium lining the folds of seminal vesicle was evident at a dose of 10mg/kg body weight of Nandrolene Decaonate. The study concluded that Nandrolene Decaonate induced alterations in the seminal vesicle