Hypertension caused by angiotensin II is dependent on vascular superoxide (O₂·−) production. The nicotinamide adenine dinucleotide phosphate (NAD[P]H) oxidase is a major source of vascular O₂·− and is activated by angiotensin II in vitro. However, its role in angiotensin II-induced hypertension in vivo is less clear. In the present studies, we used mice deficient in p47^phox, a cytosolic subunit of the NADPH oxidase, to study the role of this enzyme system in vivo. In vivo, angiotensin II infusion (0.7 mg/kg per day for 7 days) increased systolic blood pressure from 105±2 to 151±6 mm Hg and increased vascular O₂·− formation 2- to 3-fold in wild-type (WT) mice. In contrast, in p47^phox-/- mice the hypertensive response to angiotensin II infusion (122±4 mm Hg; P<0.05) was markedly blunted, and there was no increase of vascular O₂·− production. In situ staining for O₂·− using dihydroethidium revealed a marked increase of O₂·−production in both endothelial and vascular smooth muscle cells of angiotensin II-treated WT mice, but not in those of p47^phox-/- mice. To directly examine the role of the NAD(P)H oxidase in endothelial production of O₂·−, endothelial cells from WT and p47^phox-/- mice were cultured. Western blotting confirmed the absence of p47^phox in p47^phox-/- mice. Angiotensin II increased O₂·− production in endothelial cells from WT mice, but not in those from p47^phox-/- mice, as determined by electron spin resonance spectroscopy. These results suggest a pivotal role of the NAD(P)H oxidase and its subunit p47^phox in the vascular oxidant stress and the blood pressure response to angiotensin II in vivo.