We studied the influence of ventilatory frequency (1-5 Hz), tidal volume, lung volume and body position on the end-expiratory alveolar-to-tracheal pressure difference during high-frequency jet ventilation (HFJV) in Yorkshire piglets. The animals were anesthetized and paralysed. Alveolar pressure was estimated with the clamp off method, which was performed by a computer controlled ventilator and which had been extensively tested on its feasibility. The alveolar-to-tracheal pressure difference increased with increasing frequency and with increasing tidal volume, the common determinant appearing to be the mean expiratory flow. The effects in prone and in supine position were similar. Increasing thoracic volume decreased the alveolar-to-tracheal pressure difference indicating a dependence of this pressure difference on airway resistance. We concluded that the main factors determining the alveolar-to-tracheal pressure difference (delta P) during HFJV are expiratory flow (V'E) and airway resistance (R), delta P congruent to V'E x R.