To study the adaptive response of the vascular wall to blood flow changes, an arteriovenous shunt was constructed between the common carotid artery and the external jugular vein in 12 dogs. Six to eight months postoperatively, the arterial internal radius (r) was determined by angiography and/or the use of pressure-volume relationship. The results showed that r increased with increased flow load (f) and vice versa. Wall shear rate (gamma) was calculated from gamma = 4f/(tau r3), assuming laminar flow. The value of gamma, initially proportional to f, had recovered almost to the control level (within 15%) due to the vessel dilatation or atrophy during the chronic experiment, when f was less than 4 times the control. Transendothelial protein permeability, evaluated at the T-1824-stained surface by a reflectometric method, also showed a close correlation with wall shear (r = 0.934). A local autoregulatory mechanism of wall shear stress involving protein turnover in the vascular wall is suggested.