The technique of the intravenous injection of a non-diffusible radioactive indicator was studied in cats with a known acute arterial obstruction to investigate a simple method for the measurement of cerebral circulation. Acute total obstruction to flow through the right middle cerebral artery was demonstrated in all 10 animals in which it was obtained. However, the results of obstruction were obvious in only 8 of the 10. Evidence of partial obstruction was obtained in only 1 of 5 animals. Despite total obstruction, some radioactivity was recorded in the area supplied by the middle cerebral artery. This is evidence for collateral flow through leptomeningeal and cerebral anastomoses. It is doubtful that the intravenous nondiffusible indicator technique will find application in investigations of cerebrovascular physiology in animals, but further investigation in man is justifiable.