Abstract: A continuous mechanical ventilator system includes a ventilator known in the art, with a pneumatic circuit connectable to the patient and operating in the normal manner. A bag and bag valve are interposed within the pneumatic connection such that when the value is closed, the patient is on full assisted breathing from the ventilator. When the bag valve is opened, gas from the ventilator passes into the bag from which it may be drawn freely by the patient in an unassisted manner. Electronic controls provide means for varying the proportion of breaths to be assisted by the ventilator to the breaths from the bag which, of course, are unassisted. The exhalation valve normally provided, is normally operatively connected to the ventilator in the usual way, when the device is in the assisted mode. However, during the unassisted mode, the exhalation valve is isolated from the rest of the circuit so that no pressure pulses be fed to the exhalation valve during this time.