Objective: Benign prostatic hyperplasia (BPH) and erectile dysfunction (ED) are the most common illnesses in aged male patients. 5α-reductase inhibitors (5-ARIs) are suggested for the treatment of BPH. Furthermore, the association of 5ARIs with ED has been indicated. This study aimed to investigate the effect of intracavernosal injection of sildenafil on ED in 5ARI treated rats.Material and Method: Sprague-Dawley rats (n=30) were divided into three groups: Control; 10-week dutasteride treatment (0.5 mg/rat/day); and 6-week durasteride treatment followed by a 4-week washout period. In vivo erectile responses were assessed before and after intracavernosal injection of sildenafil (0.3mg/kg/rat). The relaxant and contractile responses of isolated corpus cavernosum were evaluated in in vitro organ bath.Result and Discussion: Prostate weight decreased after 10-week dutasteride treatment. In vivo erectile responses, endothelial and nitrergic relaxation responses were decreased in dutasteride groups. The washout period moderately normalized erectile responses. The intracavernosal injection of sildenafil increased erectile function in treatment groups. Contractile responses were augmented in 10-week dutasteride treated rats. The cessation of the treatment did not alter erectile function as well as endothelial relaxation and nitrergic relaxation. Also, intracavernosal sildenafil caused an improvement in 5ARI treatment-induced ED.