In order to study the ultrastructural changes of the endothelial surface induced by blood flow change, an arterio-venous shunt was constructed between the common carotid artery and the external jugular vein in 10 dogs. Blood flow rate was measured before anastomosis, after anastomosis, and after 1 week. After the final measurement (1 week), the arteries were fixed under normal pressure by 2.5% glutaraldehyde solution in phosphate buffer (pH 7.4) and were examined with scanning and transmission electron microscopes. Protrusion of nucleic parts of the endothelial cells, microvilli-like surface projections, and disturbance of pinocytotic vesicles predominated in the shunted arteries exposed to elevated blood flow. No significant ultrastructural changes could be defined in the intercellular junction of the endothelial cells which supported the increased permeability through the junction. It was suggested that the wall shear stress exerted a reactive change on the endothelial cells as well as a direct mechanical deformative effect in the endothelial plasma membrane.