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A chest-lung model, consisting of a human laryngo-tracheo-bronchial tree cast (4 or 5 bronchial generations) tightly enclosed in a 100 l rigid box was used to assess the potential efficiency of high frequency jet ventilation in patients with upper tracheal stenosis. The elasticity of the air in the box stimulated normal adult chestlung compliance. Diaphragms (0.5 or 1.5 cm thick) were inserted into the upper trachea to simulate stenoses of 0.7, 1, 1.5 and 1.75 cm inner diameter. A rigid injectorcatheter (5 mm outside diameter) was directed in the axis of the trachea with its tip 2.5 cm beneath the stenosis. The end inspiratory alveolar pressure (PA), the end expiratory pressure (PEEP) and the tidal volume (VT) were measured at a rate of 100/min and 30% inspiratory to total periods ratio. Entrained flow, Vem, measured at the start of air insufflation, was compared to that calculated (\(\dot Vec\)) from a simple model. For a given setting of the ventilator, PEEP, PA — PEEP and VT were approximately linearly related to the difference in diameters of stenosis and injector. While PEEP decreased, both PA — PEEP and VT increased with increasing diameter of stenosis. When the diameter of the stenosis was higher than 1.5 cm no changes in PA — PEEP and VT were observed, owing to the narrowest section of the larynx.\(\dot Vec\) was always higher than\(\dot Vem\). The thickness of the stenosis did not affect the results, and the diameter of the stenosis appeared to be the main factor affecting the ventilatory parameters under our experimental coditions. This suggests that a stenosis diameter under 10 mm could affect the efficiency of jet ventilation in clinical practice.