A volume ventilator is disclosed which has a balloon valve type exhalation valve for permitting release of gases to the ambient environment during the exhalation period of artificial ventilation or spontaneous breathing, or when there is excess proximal pressure in the ventilator system. A pressure transducer continuously senses the proximal pressure in the ventilator system and sends to a microcomputer controller and associated circuitry of the ventilator system a signal indicative of the instantaneous proximal pressure. The pressure in the balloon, which is the pilot pressure of the exhalation valve, is regulated through a jet venturi type pressure controller which is connected to a source of pressurized gas. The gas flows through the jet venturi, wherein its flow rate is regulated by a ball valve moving against a suitable valve seat. The ball valve is controlled by a stepper motor through a cam follower. The stepper motor is controlled through electronic signals generated in the computer control in response to the difference between the instantaneously sensed proximal pressure, and the pilot pressure required to be present in the balloon in accordance with an algorithm or preselected pilot pressure-versus-time function. Accurate servo control of the flow of respiratory gas to the patient is accomplished by a flow control subsystem which incorporates a flow control valve controlled by a second stepper motor. The flow of the respiratory gas is monitored, downstream of the flow control valve, by a flow transducer. The flow transducer sends to the microcomputer controller a signal indicative of the instantaneous flow rate of the respiratory gas. The microcomputer controller compares the instantaneous flow rate to a flow rate value required by an algorithm or preselected flow rate-versus-time function stored in its memory. The microcomputer controller then sends a signal to the second stepper motor to adjust the flow control valve to provide the flow rate instantaneously required by the preselected function or algorithm.