Pressure support ventilation (PSV) is a pressure assist form of mechanical ventilatory support that augments the patient's spontaneous inspiratory efforts with a clinician selected level of positive airway pressure. To understand the effects of PSV on respiratory function, experiments were performed on 15 stable patients requiring synchronized intermittent mandatory ventilation (SIMV), as well as on a mechanical model simulating these patients' ventilatory systems. In the clinical study, gas exchange, airway pressures, blood pressure and heart rate were measured while SIMV was replaced by enough PSV to approximate the baseline SIMV tidal volume (VT). Measurements were repeated while this PSV level was then reduced in three 5 cm H2O steps every 10 to 15 minutes. It was found that PSV was a reasonable form of mechanical ventilatory support in patients with spontaneous ventilatory drives. It improves patient comfort, reduces the patient's ventilatory work, and provides a more balanced pressure and volume change form of muscle work to the patient. The clinical significance of these properties during the weaning process remain to be determined.