Abstract: An apparatus for assisting the spontaneous breathing of a patient includes mixing unit (1) for mixing air and oxygen has an air-flow sensor (8), an electrically controlled air valve (7), an oxygen-flow sensor (9), and an electrically controlled oxygen valve (7). A respiratory-gas pressure sensor (20) is disposed on a gas duct (15) which connects the respiratory-gas unit (1) to an outlet (16). An electrically controlled outlet valve (17) is placed on the outlet (16). The air-flow sensor (8), the oxygen-flow sensor (9), the respiratory-gas pressure sensor (20), the air valve (6), the oxygen valve (7) and the outlet valve (16) are connected to an electronic control unit (40). The sensor signals produce input variables for the control unit. The control unit delivers control signals for the valves.