The effects of testosterone on skeletal muscle were assessed in adult male Wistar rats aged 80 days and 1 year. The animals were divided into 4 groups: young testosterone (YT), old testosterone (OT), young control (YC), and old control (OC) groups. The YT and OT groups received 15 applications of testosterone cypionate (5 mg/kg) on alternate days and the controls received injections containing sterile oil alone. After 30 days the animals were sacrificed and the soleus (SOL) and extensor digitorum longus (EDL) muscles were analyzed using mATPase histochemistry. After treatment, YT group gained less body weight than YC group and OT group decreased body weight, differently from the body weight gain observed in the OC group. Testosterone treatment did not show significant changes in both relative muscle weight and muscle fiber composition profile. However, in the YT group we observed an increase in the cross-sectional area of type I fibers in the SOL muscle, and type I and IIAD fibers in the EDL muscle. In the OT group, the cross sectional area of type I was decreased in the EDL muscle. These results reveal that testosterone did not cause a shift in muscle fiber type, but the cross-sectional area had fiber type-specific changes.