A hydraulic vascular model with glass and silicone tubes of the intracranial portion of the vertebro-basilar artery was used to determine the critical stenosis causing vertebrobasilar insufficiency, and the minimum diameter of the posterior communicating arteries (PComAs) necessary to tolerate therapeutic vertebrobasilar occlusion for unclippable aneurysms. The critical stenosis of one vertebral artery (VA) or basilar artery (BA) differed greatly depending upon the anatomical variations of the PComA and the posterior cerebral artery (PCA): 1.14 mm diameter when the artery supplies 80 ml/min to the cerebellum and brainstem only, 1.33 mm when 140 ml/min to these structures and one PCA, and 1.56 mm when 200 ml/min to these structures and both PCAs. The minimum PComA diameter to tolerate therapeutic occlusion depended largely upon the occlusion site: one PComA with 1.54 mm diameter for bilateral VA occlusion and 1.25 mm for BA occlusion distal to the branching of the superior cerebellar arteries. The total volume of collateral flow through both PComAs can be estimated by summing the squares of the diameters. These values cannot be applied rigidly to clinical cases, but are useful standards to evaluate the stenotic lesion or tolerance to occlusion.