The proposal of the role of mechanical forces as a localizing factor of atherosclerosis has led many researchers to investigate their effects on vascular endothelial cells. Most previous efforts have concentrated on either the fluid shear stress, which results from the flow of blood, or the circumferential "hoop" stretch, which results from the expansion of the artery during the cardiac cycle. In fact, arterial endothelial cells are subjected to both fluid shear stress and cyclic hoop stretch in vivo. Therefore, a more complete investigation of mechanical phenomena on endothelial cell behavior should include both kinds of mechanical stimuli. This study was undertaken to design an experimental apparatus that could subject cultured vascular endothelial cells to simultaneous physiologic levels of both shear stress and cyclic hoop stretch. The experimental apparatus consists of four cylindrical elastic tubes so that the following conditions may be studied: (a) static conditions: (b) shear stress only; (c) hoop stretch only; and (d) shear stress and hoop stretch. In order to establish the functional capabilities of the apparatus, bovine pulmonary artery endothelial cells were cultured in the tubes, and their morphology and f-actin structure were observed with confocal microscopy. The cells remained healthy and attached to the walls throughout the 24 hr experiment. Preliminary results indicated that the alignment of endothelial cells subjected to shear stress was significantly enhanced by the addition of hoop strain.