After reducing the number of patent conduit arteries to the brain by bilateral ligation of the carotid artery, the percentage decrease in blood pressure from the aorta to the internal carotid artery distal to the ligation was larger in spontaneously hypertensive rats than in normotensive rats. The pressure drop corresponded to the degree of hypertension as well as to morphometrically determined structural arterial alteration in the main communicating circuit, i.e. larger media to internal radius ratio and smaller internal radius in the posterior communicating arteries, the proximal part of the posterior cerebral arteries, the basilar artery and the vertebral arteries. The discrepancy between the sum of the luminal cross sectional areas of the communicating circuit and the luminal areas of the ligated conduit arteries was larger in the hypertensive than in the normotensive rats. It is to be expected that occlusion of conduit arteries to the brain will have a larger impact on the cerebral arterial perfusion pressure head in the presence of such hypertensive structural alterations known to increase flow resistance.