Several mechanisms impair cerebral vasodilatation during chronic hypertension. First, the external diameter of cerebral arterioles is reduced during chronic hypertension by structural 'remodeling'. Thus, both vascular hypertrophy and remodeling result in encroachment on the lumen. Second, endothelium-dependent dilatation of cerebral vessels is impaired during chronic hypertension. Third, blood flow through cerebral collaterals is impaired by chronic hypertension, so that an important compensatory mechanism is compromised. Impaired vasodilator responses, together with limitation of increases in collateral blood flow, may predispose to cerebral ischemia and stroke during chronic hypertension.