BACKGROUND: Increases in oxidant stress, i.e. excessive production of superoxide anion (O2(.-)), have been reported in different models of hypertension. This study was designed to test the hypothesis that increased O2(.-) production, more than diminished nitric oxide (NO) generation, plays a critical role in endothelial dysfunction present in spontaneously hypertensive rats (SHR). METHODS: The study was performed in 30-week-old normotensive Wistar-Kyoto rats (WKY) and SHR. In addition, 16-week-old SHR were treated with oral irbesartan (average dose 20 mg/kg per day) for 14 weeks (SHR-I). Aortic nicotinamide adenine dinucleotide/nicotinamide adenine dinucleotide phosphate (NADH/NADPH) oxidase activity was determined by use of chemiluminescence with lucigenin. Aortic constitutive nitric oxide synthase (cNOS) activity was determined by measuring the conversion of L-arginine to L-citrulline. Vascular responses to acetylcholine were determined by isometric tension studies. RESULTS: Whereas systolic blood pressure (SBP) was significantly increased in SHR compared with WKY, no differences were observed in SBP between SHR-I and WKY. In SHR compared with WKY, we found significantly greater NADH/NADPH-driven O2(.-) production, similar cNOS-mediated NO production and an impaired vasodilation in response to acetylcholine. Treated SHR had similar NADH/NADPH oxidase activity and significantly lower cNOS activity than the WKY group. Vasodilation in response to acetylcholine was improved in SHR-I. CONCLUSIONS: These findings suggest that a diminished availability of NO secondary to an enhanced NADH/NADPH oxidase-dependent O2(.-) production may play a critical role in endothelial dysfunction of adult SHR.