Elevation of blood homocysteine (Hcy) levels (hyperhomocysteinemia) is a risk factor for cardiovascular disorders. We previously reported that oxidative stress contributed to Hcy-induced inflammatory response in vascular cells. In this study, we investigated whether NADPH oxidase was involved in Hcy-induced superoxide anion accumulation in the aorta, which leads to endothelial dysfunction during hyperhomocysteinemia. Hyperhomocysteinemia was induced in rats fed a high-methionine diet. NADPH oxidase activity and the levels of superoxide and peroxynitrite were markedly increased in aortas isolated from hyperhomocysteinemic rats. Expression of the NADPH oxidase subunit p22 phox increased significantly in these aortas. Administration of an NADPH oxidase inhibitor (apocynin) not only attenuated aortic superoxide and peroxynitrite to control levels but also restored endothelium-dependent relaxation in the aortas of hyperhomocysteinemic rats. Transfection of human endothelial cells or vascular smooth muscle cells with p22 phox siRNA to inhibit NADPH oxidase activation effectively abolished Hcy-induced superoxide anion production, thus indicating the direct involvement of NADPH oxidase in elevated superoxide generation in vascular cells. Taken together, these results suggest that Hcy-stimulated superoxide anion production in the vascular wall is mediated through the activation of NADPH oxidase, which leads to endothelial dysfunction during hyperhomocysteinemia.