P>Erectile dysfunction (ED) is a major public health problem that seriously affects the quality of life of patients and their partners and its prevalence increases significantly with ageing. In this study, we tested the hypothesis that age-associated decrease in penile endothelial (eNOS) and neuronal nitric oxide synthase (nNOS) activity in aged rats may be increased by regular exercise. A total of 28 young (4 m) and aged (24 m) male rats were divided into four equal groups: group 1 - young control; group 2 - young trained; group 3 - old control and group 4 - old trained group. Groups 2 and 4 rats were trained to swim for 30 min a day and 5 days a week, which lasted 8 weeks. At the end of 8 weeks, rats were sacrificed and penile tissues evaluated for eNOS and nNOS activities. eNOS and nNOS activities were evaluated by immunohistochemistry in paraffinized penile tissues and results assessed semiquantitatively. Results also were compared with healthy age-matched and adult (4 m) controls. Serum level of testosterone (T) was determined using ELISA kits (Beckman Coulter, Fullerton, CA, USA). In penile tissues of aged control rats, eNOS and nNOS staining were weakly positive; however in trained groups, eNOS and nNOS immunoreactivity were increased. In young control group, eNOS and nNOS activities were more intense than aged control. eNOS and nNOS activities were higher in adult trained group than control. Serum T concentrations were significantly higher in young and aged trained group than in control groups. We can suggest that regular exercise upregulates eNOS and nNOS expressions in the aged and young rat penis. Regular exercise may improve penile erection by increasing penile neurotransmitter in both young and aged rats.