An unexpected increase of airway and alveolar pressure has been reported in recent studies of high-frequency ventilation, especially during high-frequency oscillatory ventilation. This phenomenon increases with higher respiratory frequency, driving pressure and the inspiratory time ratio(IT%) To evaluate the changes of airway and alveolar pressure, a study was done on 8 healthy cats with high frequency jet ventilation (HFJV). The results are as follows. On fixed driving pressure at 1 psi: 1) Adequate ventilation occurred in 20% inspiratory time % during a frequency of 1 Hz or 2Hz, but did not occur during a frequency of 3 Hz and IT% 10% 2) The alveolar pressure and airway pressure increased with an increase of respiratory rate and IT% 3) The alveolar pressure was lower than the mean airway pressure, but higher than the end diastolic airway pressure. Accordingly, "auto-PEEP" phenomenon was observed. However, the value of auto-PEEP did not significantly vary with the changes of respiratory rate and IT%. In conclusion, under the constant driving pressure (1 psi), mean airway pressure and alveolar pressure was correlated with the respiratory rate and inspiratory time/cycle%(IT%) and auto-PEEP phenomenon was observed during high-frequency jet ventilation on healthy cats.