Abstract: Two identical bag-in-bottle bellows assemblies are provided for separately receiving and discharging breathable oxygen/air and oxygen/air/Xenon gas mixtures. An adjustable piston/cylinder gas pump is connected with the bellows assemblies through a system of conduits and valves for selectively operating the bellows of either assembly. Hoses are interconnected between the bellows and a patient mask to form inspiratory and expiratory paths. The expiratory path includes a PEEP valve, a pressure actuated exhalation valve, an O.sub.2 concentration analyzer, and a CO.sub.2 absorber. The apparatus is capable of being instantaneously switched between the isolated oxygen/air and oxygen/air/Xenon delivery systems while continuously ventilating the patient and maintaining ventilation parameters (ventilation rate, inspiratory time, oxygen concentration, end-expiratory pressure, and tidal volume) substantially constant.