Abstract. The relationships between fluid shear stress, a physiologically relevant mechanical force in the circulatory system, and pinocytosis (fluid-phase endocytosis) were investigated in cultured bovine aortic endothelial cells using a specially designed apparatus. Continuous exposure to steady shear stresses ( 1-15 dyn/cm2) in laminar flow stimulated time-and amplitude-dependent increases in pinocytotic rate which returned to control levels after several hours. pendent and shows accommodation, (c) certain patterns of fluctuation in shear stress result in sustained elevation of pinocytotic rate, and (d) shear stresses can modulate endothelial pinocytosis independent of growth stimulation. These findings are relevant to (i) transendothelial transport and the metabolism of macromolecules in normal endothelium and (ii) the role ofhemodynamic factors in the localization of atherosclerotic lesions in vivo.