This study aims to determine the effect of shear stress on membrane potassium (K\sp+) permeability in vascular endothelial cells. Cultured monolayers of calf pulmonary artery endothelial cells, preloaded with \sp86Rb\sp+, were subjected to different levels of fluid shear stress in the range 1-10 dynes/cm\sp2, in a parallel-plate geometry flow chamber, and the radioactivity of the effluent was monitored with time. Increase in laminar shear stress, from 1 dyne/cm\sp2 to a higher level, resulted in a rapid transient increase in the rate constant for \sp86Rb\sp+ release. The difference of efflux rate coefficients, between the peak and the baseline at 1 dyne/cm\sp2, varied with shear stress in a "dose-dependent" manner. Changes in K\sp+ permeability may occur via activation of shear-stress-activated K\sp+ channels, Ca\sp2+-activated K\sp+ channels, or both. The Ca\sp2+-activated K\sp+ channels were shown to respond to bradykinin stimulation under flow conditions. The significance of such flow studies is that they provide more knowledge about the flow-associated changes in ionic channels and intracellular second messengers in endothelial cells.