To observe cytoplasmic microfilaments in the endothelial cells of flow-loaded arteries, an arteriovenous shunt was constructed between the common carotid artery and the external jugular vein in 26 dogs. After measuring the flow rates of the arteries, the endothelial layer was examined ultrastructurally with a transmission electron microscope at three different times: 1 week (acute experiments), 2-4 weeks (subacute experiments), and 4-7 months (chronic experiments). Six-to seven-nanometer microfilaments were found forming bundles, which usually ran longitudinally along the long axis of the vessel. In the acute experiments, the bundles increased in the endothelial cells of the flow-loaded arteries. They showed incomplete striation and were mostly located close to the basal cell membrane. In the subacute experiments, they showed an increase with the development of cross-striation. The half-desmosomal structure of the basal cell membrane had developed a close connection to the bundles. In the chronic experiments, the bundles were especially conspicuous around the intercellular junction. Tennanometer microfilaments increased in the endothelial cells of the flow-loaded artery in the subacute and chronic experiments. We consider that the bundles of 6- to 7-nm microfilaments might be structures developed to combat wall shear stress corresponding to actin filament stress fibers.