NADPH oxidase plays an important role in vascular oxidative stress in hypertensive diseases. We evaluated whether NADPH oxidase-dependent superoxide (O 2 − ) production is involved in the deoxycorticosterone acetate (DOCA)-salt-induced hypertension, using mice which are genetically deficient in gp91phox, an NADPH oxidase subunit protein (gp91 −/− mice). Two weeks after the DOCA-salt treatment, systolic blood pressure was significantly elevated in wild-type mice, but not in gp91 −/− mice. After a 5-week treatment period, wild-type mice developed high blood pressure, with a systolic blood pressure of 127±3 mm Hg, compared with 107±4 mm Hg in gp91 −/− mice. Aortic O 2 − production in wild-type DOCA-salt-treated mice was significantly higher than that in wild-type sham mice, whereas there were no significant differences in aortic O 2 − production between gp91 −/− DOCA-salt-treated and sham mice. These findings suggest that vascular O 2 − overproduction via gp91phox-containing NADPH oxidase is one of the crucial factors in the development of DOCA-salt-induced hypertension.