The change in global and regional cerebral circulation after intravenous administration of pyritinol was investigated in 14 patients with acute or subchronic cerebral ischemia. The measurement of the cerebral circulation was performed by intraarterial isotope clearance with 133xenon using a multidetector apparatus. With a single administration of pyritinol (400 mg) there was a statistically significant increase in the cerebral circulation in the gray matter by 6.7 ml/100 g/min corresponding to 9.7% (p less than 0.01) 10 minutes after the end of the drug injection. In the areas of all portions of the brain with defective circulation there was a significant increase in blood flow by 8.9, 0.8 and 3.2 ml/100 g/min (gray matter, white matter, total substance), corresponding to 12.3%, 4.4% and 8.1%. In areas with a normal initial status, the increase in blood flow attained statistical significance in the gray matter only (p less than 0.05).