A preliminary design study of the nuclear gas engine, consisting of a gas-cooled reactor directly coupled to a reciprocating engine, is presented. The principles of operation of the proposed gas engine are outlined and typical variations anre discussed. The nuclear gas engine is compared with other reciprocating engines and air compressors. A comparison between the ideal and actual cycles is made, with particular attention given to pumping, heat, and other losses to be expected. The applications and development of the nuclear gas engine are discussed. (W.D.M.)