The purpose of this study was to investigate the mechanisms that regulate superoxide (O₂˙‾) production, as a function of an acute elevation of intravascular pressure and age. Mesenteric arteries isolated from young (6 month) and aged (24 month) male Fischer 344 rats were used. O₂˙‾ production in vessels in response to 80 (normal pressure, NP) and 180 (high pressure, HP) mmHg was determined by the SOD-inhibitable nitroblue tetrazolium (NBT) reduction assay. In vessels exposed to NP, O₂˙‾ production was significantly higher in aged than in young vessels (32.7±7.0 vs. 15.4±2.4 nmol/mg/30min). HP enhanced O₂˙‾ production in vessels of both groups, but the enhancement was significantly greater in aged than in young vessels (63.4±6.7 vs 32.7±4.3 nmol/mg/30min). Apocynin (100 µmol/L) attenuated HP-induced increases in superoxide production in both groups, while allopurinol (100 µmol/L) and L-NAME (100 µmol/L) inhibited the response only in aged vessels. Confocal microscopy showed increases in O₂˙‾ in response to HP in endothelial and smooth muscle layers of both groups, with much greater fluorescent staining in aged than in young rats and in the endothelium than in smooth muscle cells. No significant changes in NAD(P)H oxidase gene and protein expressions were observed in vessels of the two groups. Upregulation of protein expression of xanthine oxidase was detected in aged vessels. We conclude that NAD(P)H oxidase contributes importantly to HP-induced enhanced O₂˙‾ production in vessels of both young and aged rats, while xanthine oxidase and NOS-dependent O₂˙‾ production also contribute to the enhancement in mesenteric arteries of aged rats.