Measurements of cerebral blood flow using the nitrous oxide method were performed in 44 normal subjects and in 226 patients with apoplexy. In the apoplectic patients, the interval from the last onset of stroke varied from fourteen days to eight years. Cerebral blood flow in the patients with apoplexy was reduced to less than 80% of that in normal subjects. In the group of subacute stage, cerebral blood flow was more reduced than that of chronic stage. According to the mean arterial blood pressure at the examination, the values for cerebral blood flow were divided into 7 classes from 80 to 150mm Hg at an interval of 10mm Hg. The average value was calculated in each class. As a result, it was found that the cerebral blood flow was maintained relatively constant at the blood pressure within 80 to 150mm Hg. This may suggest the existence of autoregulation of cerebral blood flow. It was also found that there was no influence of aging on the relationship between the cerebral blood flow and the arterial blood pressure.