In high-frequency oscillatory ventilation (HFOV), an adequate mean airway pressure is crucial for successful ventilation and optimal gas exchange, but air trapping cannot be detected by the usual measurement at the y piece. Intratracheal pressures produced by the high-frequency oscillators HFV-Infantstar (IS), Babylog 8000 (BL), and the SensorMedics 3100A (SM) [the latter with either 30% (SM₃₀) or 50% (SM₅₀) inspiratory time] were investigated in four anesthetized tracheotomized female piglets that were 1 day old and weighed 1.6-1.9 kg (mean 1.76 kg). The endotracheal tube was repeatedly clamped while the piglets were ventilated with an oscillation frequency of 10 Hz, and the airway pressure distal of the clamp was recorded as a measure of average intrapulmonary pressure during oscillation. Clamping resulted in a significant decrease of mean airway pressure when the piglets were ventilated with SM₃₀ (0.86 cmH₂O), BL (0.66 cmH₂O), and IS (0.71 cmH₂O), but airway pressure increased by a mean of 0.76 cmH₂O with SM₅₀. Intratracheal pressure, when measured by a catheter pressure transducer at various oscillation frequencies, was lower than at the y piece by 0.4-0.9 cmH₂O (SM₃₀), 0.3-3 cmH₂O (BL), and 1-4.7 cmH₂O (IS) but was 0.4-0.7 cmH₂O higher with SM₅₀. We conclude that the inspiratory-to-expiratory time (TI/TE) ratio influences the intratracheal and intrapulmonary pressures in HFOV and may sustain a mean pressure gradient between the y piece and the trachea. A TI/TE ratio < 1:1 may be useful to avoid air trapping when HFOV is used.