This study tested the hypothesis that superoxide levels are elevated in isolated mesenteric resistance arteries (100–300 µm) from rats fed a short-term high-salt (HS) diet (4% NaCl for 3 days) compared to controls fed a low-salt (LS) diet (0.4% NaCl). Vascular relaxation induced by the superoxide dismutase mimetic tempol (4-hydroxytetramethylpiperidine-1-oxyl), the NADPH oxidase inhibitor apocynin and the xanthine/xanthine oxidase inhibitor oxypurinol was significantly larger in mesenteric arteries from animals fed HS diet compared to arteries from animals fed LS diet. Basal superoxide levels assessed via dihydroethidine (DHE) fluorescence were significantly elevated in arteries from rats fed HS diet, and were reduced by tempol, apocynin and oxypurinol, but not by L-NAME. Basal and methacholine-induced NO production (assessed by DAF-2T fluorescence) was significantly reduced in arteries from rats fed HS diet versus arteries from rats on LS diet. Impaired methacholine-induced NO release and vascular relaxation were restored by tempol and apocynin, but not by oxypurinol. These data suggest that the elevated production of superoxide by NADPH oxidase and xanthine/xanthine oxidase contribute to elevated basal superoxide levels, reduced NO release and impaired vascular relaxation in mesenteric resistance arteries of rats fed HS diet.