Specific NAD(P)H oxidase complexes are major sources of superoxide anion formation and oxidative stress in endothelial cells. Whether these complexes affect vascular function is not well understood. Therefore, we compared mRNA expression of NAD(P)H oxidase subunits by real time PCR, superoxide anion formation by lucigenin-enhanced chemiluminescence (5 mM), and vascular function by acetylcholine-mediated vasodilatation in phenylephrine-preconstricted aortic rings between C57/BL6 wildtype (WT) and Nox2 knockout mice. Aortic Nox4 (WT: 961.093±122.130 RU; Nox2 knockout: 936.902±46.097 RU) and p22phox mRNA expression was not altered in Nox2 knockout mice. The superoxide anion formation in the aorta was 54±5% of total radical formation in WT (n=8) and 47±5% in Nox2 knockout (n=12) animals (not significant). However, vascular function was significantly improved in aortic rings of Nox2 knockout mice. Maximal relaxation reached 51.9±6.4% of phenylephrine-preconstricted values in WT (n=37), and 22.4±6.8% in Nox2 knockout mice (n=25) (P<0.05). In conclusion, vascular function is improved in aortic rings of Nox2 knockout mice. This functional improvement is not mediated by compensatory mechanism involving Nox4 mRNA expression.