An understanding of the cerebral circulation is so fundamental to comprehension of the pathogenesis of stroke that cerebral blood flow and metabolism merit review in this series of reports. The authors recognize that the research described here is very technical in nature and may appear to have little practical application to clinical medicine. Nevertheless, these matters are basic to the development of precise methods for the measurement of regional cerebral blood flow in man which could be used to monitor the therapy of stroke with greater success than is possible at present.