The aim of this study was to examine the ability of H2S, released from NaHS to protect vascular endothelial function under conditions of acute oxidative stress by scavenging superoxide anions (O2(-)) and suppressing vascular superoxide anion production. O2(-) was generated in Krebs' solution by reacting hypoxanthine with xanthine oxidase (Hx-XO) or with the O2(-) generator pyrogallol to model acute oxidative stress in vitro. O2(-) generation was measured by lucigenin-enhanced chemiluminescence. Functional responses in mouse aortic rings were assessed using a small vessel myograph. NaHS scavenged O2(-) in a concentration-dependent manner. Isolated aortic rings exposed to either Hx-XO or pyrogallol displayed significantly attenuated maximum vasorelaxation responses to the endothelium-dependent vasodilator acetylcholine, and significantly reduced NO bioavailability, which was completely reversed if vessels were pre-incubated with NaHS (100 μM). NADPH-stimulated aortic O2(-) production was significantly attenuated by the NADPH oxidase inhibitor diphenyl iodonium. Prior treatment of vessels with NaHS (100 nM-100 μM; 30 min) inhibited NADPH-stimulated aortic O2(-) production in a concentration-dependent manner. This effect persisted when NaHS was washed out prior to measuring NADPH-stimulated O2(-) production. These data show for the first time that NaHS directly scavenges O2(-) and suppresses vascular NADPH oxidase-derived O2(-) production in vitro. Furthermore, these properties protect endothelial function and NO bioavailability in an in vitro model of acute oxidative stress. These results suggest that H2S can elicit vasoprotection by both scavenging O2(-) and by reducing vascular NADPH oxidase-derived O2(-) production.