Atherosclerotic plaques preferentially develop in regions exposed to a low mean shear stress and cyclic reversal of flow direction (oscillatory flow). This contrasts with plaque-free zones where endothelial cells are exposed to unidirectional flow. Previous works from our laboratory using a unique experimental flow system demonstrated the existence of a differential regulation of endothelial nitric oxide synthase (NOS III) gene expression by unidirectional and oscillatory flow patterns. We further studied the possible mechanisms responsible for selective unresponsiveness of NOS III gene regulation to oscillatory flow. The results obtained demonstrate that (i) induction of the activity of the 1600-base-pair NOS III gene promoter by unidirectional and oscillatory shear stress is modulated by similar mechanisms that involve NF-kappaB activation, but do not involve Ras-dependent MAP kinase activation, and (ii) the lack of induction of NOS III gene regulation by oscillatory shear stress can be attributed to the activation of a yet unidentified negative cis-acting element present in the NOS III gene.