Abstract: A volume-cycled lung ventilator providing for both spontaneous breathing of a patient and intermittent mandatory ventilation, the invention provides two parallel inspiratory flow paths, one flow path including a demand valve for supplying gas to the patient during spontaneous breathing and the other flow path including a solenoid-operated inspiratory flow valve for providing intermittent mandatory breaths. The invention provides structure capable of adjusting the intervals between mandatory breaths, the volume of each such breath and the flow waveform of each such breath. An expiratory flow path including a fluid operated expiratory flow valve is further provided. A flow rate signal generator in the expiratory flow path acts to sense the rate of exhalation of the patient and is effective to inhibit the opening of the inspiratory flow valve unless the rate of exhalation is below a predetermined value.