Distensibility of human cerebral arteries was compared quantitatively with extracranial arteries using a parameter which was obtained from the pressure-radius data. It was found that cerebral arteries were much stiffer than the extracranial arteries of comparative sizes. The stifiness of cerebral arteries reached nearly maximum at the middle age and did not progress thereafter. Effect of arterial stiffness on blood flow was also examined using pulsatile flow in polymer tubes with different distensibility under the influence of sinusoidally oscillating pressure. The stiffer the tube was, the higher was pulsatile pressure. It is concluded that much larger fluctuation of pressure may occur in the cerebral arteries compared with extracranial arteries and induce the degenerative change or structural weakness in the wall, especially at the apex of bifurcation, which may play important roles in the pathogenesis of cerebral aneurysms and the mechanism of their growth and rupture.