A system has been developed for subjecting endothelial cell monolayers to prolonged steady flow while maintaining normal culture conditions. Cloned bovine endothelial cells were grown to confluence on one wall of a square glass tube which was then incorporated in the flow circuit. Flow rates of 19-21 ml/min were sustained for periods of 6-45 hr, subjecting the cells along the center line of the wall of the tube to a maximum shear stress of 34 dyn/cm2. The cells in all the experiments remained attached and viable when subjected to this shear stress. Photographic data from experimental runs were qualitatively assessed for changes in cell morphology, confluence, and orientation and were compared to data from matched stationary controls. Five experiments were chosen for quantitative morphometric analysis. In three experiments, the cells showed elongation with their long axes aligned with the direction of flow in 6.5, 21, and 22 hr. In the other experiments, either the cells formed a swirling pattern or no change in morphology was apparent. Although cell shape (form) changed in response to shear stress, cell area remained unaffected by exposure to flow.