An experimental rotating fan-beam (3rd generation) CT scanner has been designed and constructed at The Ontario Cancer Institute. The scanner incorporates an array of 201 xenon ionization detectors. These operate at a moderate pressure of 5 atm. with a collecting potential of 1 kV. A digitizing charge integration circuit is coupled to the collecting electrode of each detector so that detector data.can be digitally multiplexed onto a computer bus. The scanner is capable of acquiring scan data in 10 seconds per tomographic slice. The experimental nature of this scanner allows flexibility of x-ray technique, data accumulation, reconstruction algorithms, and image manipulation. The design of the detector and associated electronics is described and the method of scanner calibration is outlined with emphasis on some of the problems associated with fan-beam scanners.