Abstract —Atherosclerotic lesions are found opposite vascular flow dividers at sites of low shear stress and oscillatory flow. Since endothelial proinflammatory genes prominent in lesions are regulated by oxidation-sensitive transcriptional control mechanisms, we examined the redox state of cultured human umbilical vein endothelial cells after either oscillatory or steady laminar fluid shear stress. Endothelial oxidative stress was assessed by measuring activity of the superoxide (O ₂ · ⁻ )–producing NADH oxidase (a major source of reactive oxygen species in vascular cells), intracellular O ₂ · ⁻ levels, induction of the redox-sensitive gene heme oxygenase-1 (HO-1), and abundance of Cu/Zn superoxide dismutase (Cu/Zn SOD), an antioxidant defense enzyme whose level of expression adapts to changes in oxidative stress. When cells were exposed to oscillatory shear (±5 dyne/cm ² , 1 Hz) for 1, 5, and 24 hours, NADH oxidase activity and the amount of HO-1 progressively increased up to 174±16% ( P <0.05) and 505±111% ( P <0.05) versus static conditions, respectively, whereas levels of Cu/Zn SOD remained unchanged. This upregulation of HO-1 was completely blocked by the antioxidant N -acetylcysteine (NAC, 20 mmol/L). In contrast, steady laminar shear (5 dyne/cm ² ) induced NADH oxidase activity and NAC-sensitive HO-1 mRNA expression only at 1 and 5 hours, a transient response that returned toward baseline at 24 hours. Levels of Cu/Zn SOD mRNA and protein were increased after 24 hours of steady laminar shear. Furthermore, intracellular O ₂ · ⁻ , as measured by dihydroethidium fluorescence, was higher in cells exposed to oscillatory than to laminar shear. These data are consistent with the hypothesis that continuous oscillatory shear causes a sustained activation of pro-oxidant processes resulting in redox-sensitive gene expression in human endothelial cells. Steady laminar shear stress initially activates these processes but appears to induce compensatory antioxidant defenses. We speculate that differences in endothelial redox state, orchestrated by different regimens of shear stress, may contribute to the focal nature of atherosclerosis.