A computer based multidetector system was developed for the measurement of regional cerebral blood flow (rCBF) by the 133Xenon inhalation technique. The computer portion of the system is characterized by on-line data acquisition, rapid data analysis and presentation of results. Special system features include programs which automatically determine end-tidal Xenon concentrations during the saturation and desaturation phases of the flow measurement and which provide for automatic background subtraction, and a special purpose counter-digital multiplexor-computer interface. The system provides convenience in handling acutely ill patients, and assures reproducible positioning for serial measurements. Typical output format and partial results from 150 patient and normal subject measurements are presented. © 1977.