A respiration unit for supplying a preselected limited volume of breathing gas to a patient at limited positive pressures, and at a preselected limited flow rate is disclosed. The system includes an air delivery cylinder having a drive weight which is free to move up and down. A diaphragm connected to the weight divides the cylinder into an upper control chamber and a lower delivery chamber. A vacuum source is connected to the control chamber through a first solenoid-controlled valve to regulate the lifting of the weight, and a second solenoid-controlled valve regulates the bleeding of air into the control chamber to release and regulate the rate of fall of the weight, thereby driving breathing gas under pressure out of the lower chamber. A control system regulates the lift and lower cycles of the drive weight to provide the required operation of the system, with various sensors being provided to allow patient control of the system under selected conditions, and to assure patient safety.The control system incorporates, among other things, means for providing a variable inspiratory-expiratory ratio, for permitting patient assisted breathing, for mixing oxygen with air or anesthetic gases, for periodically altering the breathing pattern, for permitting selection of positive or negative end expiratory pressure, for allowing selection of the breathing waveform, and for providing accurate displays of essential parameters.