The development of atherosclerosis is thought to be initiated by a dysfunctional state of the vascular endothelium. The proposal that mechanical forces play a role in the localization of this disease has led researchers to develop in vitro models to assess their effects on cultured endothelial cells. The arterial endothelium is exposed simultaneously to circumferential hoop stretch and wall shear stress, yet previous investigations have focused on the isolated effects of either cyclic stretch or shear stress. The influence of physiological levels of combined shear stress and hoop stretch on the morphology and F-actin organization of bovine aortic endothelial cells was investigated. Cells subjected for 24 hours to shear stresses higher than 2 dyne/cm2 or to hoop stretch greater than 2% elongated significantly compared with unstressed controls and oriented along the direction of flow and perpendicular to the direction of stretch. Exposure to more than 4% stretch significantly enhanced the responses to shear stress. Both shear stress and hoop stretch induced formation of stress fibers that were aligned with the cells' long axes. Simultaneous exposure to both stimuli appeared to enhance stress fiber size and alignment. These results indicate that shear stress and hoop stretch synergistically induce morphological changes in endothelial cells, which suggests that circumferential strain might modulate sensitivity of endothelial cells towards shear stress.