To estimate microfilament bundles in the endothelial cells of flow loaded arteries quantitatively, blood flow change was produced in the common carotid arteries of eleven female beagle dogs (1-2 years of age, 8-10 kg) employing the arteriovenous shunt method between the common carotid artery and external jugular vein. After 1 week, the amount of microfilament bundles was measured with a point counting method on transmission electron microscopical photographs. In the arteries loaded by highly elevated blood flow, the average thickness of microfilament bundles increased over 0.1 mu m (non-operative control: 0.01-0.02 mu m) and the average longitudinal cut area of a microfilament bundle grew over 0.7 mu m2 (non-operative control: 0.2 mu m2). The number of bundles increased over 1.0 piece (non-operative control: 0.2 pieces) per unit length (mu m) in transverse section and increased over 0.5 pieces (non-operative control: 0.2 pieces) in longitudinal section. The average thickness of the bundles significantly correlated to the blood flow increase ratio (the ratio of the blood flow rate of the final measurement to that before anastomosis). We consider that the microfilament bundles are the structure developed to combat the wall shear stress induced by the elevated blood flow.