The performance of a commercially available oscillator (SensorMedics 3100) at different frequencies was assessed. A frequency response curve of a pneumotachograph system was constructed and this was used to measure the volume delivered by the oscillator to a lung model. The volume delivered by a constant diaphragm displacement was demonstrated to be inversely proportional to the frequency, but unaffected by increasing mean airway pressure from 15 to 25 cm H2O. The volume delivered during high frequency oscillation (HFO) was then assessed in 8 infants, median gestational age 29 weeks. The infants were studied at two frequencies, 10 and 15 Hz, both of which were used at two levels of mean airway pressure (MAP): 2 and 5 cm H2O above the MAP level previously used during conventional ventilation. The delivered volume was not significantly different at the two MAP levels, but was significantly greater at 10 than 15 Hz at both MAP levels (P < 0.03); at MAP +2 cm H2O above baseline the reduction in delivered volume was from a median of 1.54 mL/kg (range, 0.88-3.12) at 10 Hz to 1.18 mL/kg (range, 0.65-4.5) at 15 Hz. These results suggest that higher frequencies would require an increase in the oscillator displacement if effective gas exchange is to be maintained.