We tested two hypotheses, first that exercise training reverses age-related decrements in endothelium-dependent dilation in soleus muscle feed arteries (SFA) and second that this improved endothelium-dependent dilation is the result of increased nitric oxide (NO) bioavailability due to increased content and phosphorylation of eNOS and/or increased antioxidant enzyme content. Young (2 mo) and old (22 mo) male Fischer 344 rats were exercise trained (Ex) or remained sedentary (Sed) for 10-12 wks yielding four groups of rats: 1) young Sed (4-5 mo), 2) young Ex (4-5 mo), 3) old Sed (24-25 mo), and 4) old Ex (24-25 mo). SFA were isolated and cannulated with two glass micropipettes for examination of endothelium-dependent [acetylcholine (ACh)] and endothelium-independent [sodium nitroprusside (SNP)] vasodilator function. To determine the mechanism(s) by which exercise affected dilator responses, ACh-induced dilation was assessed in the presence of N-nitro-L-arginine (L-NNA; to inhibit NO synthase), indomethacin (Indo; to inhibit cyclooxygenase), and L-NNA + Indo. Results indicated that ACh-induced dilation was blunted in old Sed SFA relative to young Sed SFA. Exercise training improved ACh-induced dilation in old SFA such that vasodilator responses in old Ex SFA were similar to young Sed and young Ex SFA. Addition of L-NNA, or L-NNA + Indo, abolished the exercise effect. Immunoblot analysis revealed that extracellular superoxide dismutase (SOD) protein content was increased by training in old SFA whereas eNOS and SOD-1 protein content were not altered. Addition of exogenous SOD, or SOD + catalase, improved ACh-induced dilation in old Sed SFA such that vasodilator responses were similar to young Sed SFA. L-NNA abolished the effect of exogenous SOD in old Sed arteries. These results indicate that exercise training reverses age-induced endothelial dysfunction in SFA by increasing NO bioavailability and that increases in vascular antioxidant capacity may play an integral role in the improvement in endothelial function.