Abstract: A breathing gas system, primarily intended for use during anaesthetization of patients, has a respiratory circuit, a drive gas apparatus and a pressure-transmitting bellows device, connected by a first space to the respiratory circuit and by a second space to the drive gas apparatus. When an outlet valve is regulated at the end of an expiratory phase to minimize the pressure difference between the first space and the second space, the respiratory circuit responds immediately at the next inspiratory phase when drive gas is sent to the second space. Minimizing the pressure difference also makes possible use of the pressure-regulated operating mode with the breathing gas system, and a rapid response to attempts at spontaneous breathing by the patient is achieved.