The distensibility of cerebral vessels, defined as the percent increase in cerebrovascular volume per mmHg rise of arterial blood pressure, was in-vestigated in 23 cats using our recently developed photoelectric apparatus. The cerebral vasomotor tone was reduced by one of the following procedures: CO₂ inhalation, brain damage, or intravenous administration of tetrodotoxin or vasodilators. To distend the cerebral vessels, the systemic arterial blood pressure (SABP) was elevated by intravenous administration of 1-nor-adrenaline. It was found tat intact cerebral vessels showed no change or even a slight decrease in distensibility regardless of the level of the initial pressure within the range of autoregulation. On the other hand, vessels which had previously experienced one of the above affronts except for CO₂ inhalation tended to show an increased value of distensibility. The magnitude of the distensibility appeared to be related to the initial level of SABP. Loss or decrease of tonic contraction of arterioles in response to a sudden rise in SABP leads to a total or partial impairment of the autoregulation of blood flow. In the latter case, preservation of the autoregulatory mechanism in the direction of lowered blood pressure within the limits of the autoregulatory range does not necessarily imply its operation in the opposite direction. Maximum distensibility was demonstrated in papaverine-pretreated cats, causing either an increase in flow with a closed skull or transcalvarial hernia-tion with an open skull. The brain swelling was reproducible, and of purely vascular origin.