Ultrastructural observation of the elastic fibers in the flow loaded canine carotid arteries was performed by employing a specific elastic fiber stain for electron microscopy. Experiments: Arterio-venous shunt was constructed between the right common carotid artery and the right external jugular vein in six adult beagle dogs. Blood flow rates through the right common carotid artery and the left common carotid artery before anastomosis, after anastomosis and before sacrifice were used for the index of the blood flow loading. After 6 to 8.5 months, these common carotid arteries were observed by transmission electron microscope after staining with tannin acid uranyl acetate and lead citrate. Results : The general structure of the shunted arteries loaded high blood flow was preserved, although they showed slightly larger diameter than the control arteries. The elastic fibers of the media were increased quantitatively in the flow loaded shunted arteries as compared to those in the control arteries. Consideration: Chronic blood flow load would induce increase and growth of the elastic fibers in the media of the canine carotid artery. We consider that these changes would be mostly physiologic occurrence in the course of the adaptive response. (J. Jpn. Soc. Biorheol., 1 (1), 19~26, 1987).