In order to demonstrate that IL-8 mRNA expression in endothelial cells is not only regulated by chemical factors, but also by mechanical factors, in this article, after pretreating cultured human umbilical vein endothelial cells (HUVECs) with shear stress for different time, we employed both RT-PCR to assay IL-8mRNA expression and immunocytochemical staining to detect NF-kB activation in HUVECs. We found that: (i) IL-8mRNA expressed little in HUVECs untreated or pretreated with low laminar shear stress for 0.5 hour; IL-8mRNA expression was increased when HUVECs were pretreated with low laminar shear stress for 1 hour, and increased further when pretreated for 2 hours; (ii) the immunoreactivity of NF-kB p65 in the nuclei of HUVECs untreated or pretreated with low laminar shear stress for 0.5 hour was negative, while it became weak positive in the nuclei of HUVECs pretreated with shear stress for 1 hour and positive in the nuclei of HUVECs pretreated for 2 hours. The results imply that low laminar shear stress was capable of inducing IL-8 gene expression and activating NF-kB, which were both time-dependent. The induction of IL-8 gene expression by laminar shear stress is probably due to the activation of NF-kB. We suggest that IL-8mRNA expression in endothelial cells induced by low shear stress may play a key role in the pathogenesis and development of both inflammation and arterioatherosclerosis.