A method is described for quantifying regional cerebral blood flow (rCBF), oxygen extraction (rOER), and oxygen utilization (rCMRO2) in man non-invasively, using inhaled 15O-labeled CO2 and O2 and positron emission tomography. The theoretical considerations and practical aspects are described. The results of rCBF, rCMRO2, and rOER for gray and white matter in normal volunteers are presented. The significance and limitations of the results are discussed. The method appears suitable for the study of human cerebral aerobic metabolism and blood flow under differing physiological conditions and in a wide variety of neurological disorders.