Endotoxin (or lipopolysaccharide, LPS) increases levels of superoxide in blood vessels and impairs vasomotor function. Angiotensin II (AII) plays an important role in generation of superoxide in several disease states, including hypertension and heart failure. The goal of this study was to determine whether activation of the renin-angiotensin system contributes to oxidative stress and endothelial dysfunction after endotoxin. We examined effects of enalapril (an angiotensin-converting enzyme inhibitor, ACEI), or L158809 (an angiotensin receptor blocker, ARB), on increases of superoxide and vasomotor dysfunction in mice treated with LPS. C57/BL/6 mice were treated with either enalapril (60 mg/kg/day) or L158809 (30 mg/kg/day) for 4 days. After the third day, LPS (10-20 mg/kg) or vehicle was injected i.p., and one day later, vasomotor function of aorta was examined in vitro. After precontraction with PGF₂, maximal responses to sodium nitroprusside were similar in aorta from normal and LPS-treated mice. In contrast, relaxation to acetylcholine was impaired after LPS (54±5% at 10^-5, mean±SE) compared to vessels treated with vehicle (88±1%; p<0.05). Enalapril improved (p<0.05) relaxation in response to acetylcholine to 81±6% after LPS. L158809 also improved relaxation in response to acetylcholine to 77±4% after LPS. Superoxide (measured with lucigenin and hydroethidine) was increased (p<0.05) in aorta after LPS, and levels were reduced (p<0.05) following enalapril and L158809. Thus, after LPS, enalapril and L158809 reduce superoxide levels and improve relaxation to acetylcholine in the aorta. The findings suggest that activation of the renin/angiotensin system contributes importantly to oxidative stress and endothelial dysfunction after endotoxin.