The effect on tidal volume and airway pressure of increasing ventilator rate (30, 60, and 120/min) was tested in six commonly used neonatal ventilators. In all six ventilators increased flow was necessary to maintain mean airway pressure at the higher rates. Tidal volume decreased at rates of both 60 and 120/min in all six ventilators, associated with a change in pressure waveform. The most marked reduction in tidal volume, however, was associated with increased positive end-expiratory pressure (PEEP). This was only demonstrated in four ventilators, all incorporating nonassisted expiratory valves. These results stress the necessity for appropriately designed ventilators if fast rates are to be used routinely in clinical practice.