A pressure-limited, time-cycled pulmonary ventilator includes a pressurized gas system that supplies respiratory gas to a patient at a selected inspiratory flow rate; an exhalation valve that (a) opens an expiratory flow path in response to the elapsing of a selected inspiratory time period, and (b) maintains a selected proximal pressure limit during the inspiratory time period; and a volume-cycle override system that actuates the exhalation valve so as to open the expiratory flow path if the patient's tidal volume reaches or exceeds a selected volume limit during the inspiratory time period. The volume-cycle override system includes a flow sensor that outputs an inhalation flow rate signal. The flow rate signal is integrated to derive a measured tidal volume signal. An operator control produces a volume limit signal, indicating a selected volume limit, that is compared with the measured tidal volume signal to yield a difference signal. The exhalation valve opens the expiratory flow path in response to the difference signal having a value indicating that the measured tidal volume is equal to or greater than the selected volume limit, as long as the proximal pressure exceeds a selectable reference pressure. An exhalation control system provides a first reference pressure at the beginning of the inspiratory time period, and a second reference pressure at the end of the inspiratory time period, or when the difference signal indicates that the measured tidal volume is equal to or greater than the selected volume limit, whichever first occurs.