Air entrainment contribution to jet-ventilation during bronchoscopy was evaluated as a lung model (increasing compliance and airway resistance). Ventilation was provided through a 10 mm internal diameter tube using either jet alone without air entrainment, or injection with air entrainment (coaxial and lateral injectors). Three I/E ratios (0.25, 0.43 and 0.67) and nine rates of ventilation, ranging from 20 to 300 c X min-1, were assessed. The driving pressure of the injected air was 350 +/- 10 kPa. Air entrainment is an important part of total ventilation (63.7 +/- 5.5%). The magnitude of air entrainment depends upon the levels of peak and end expiratory airway pressures. Increase in lung volume varies linearly with the end expiratory pressure. Air entrainment contributes to keep the tidal volume above the model dead-space. CO2 elimination is related to the magnitude of ventilated volumes. The amount of entrained air interferes with the FIO2 of delivered gases. During bronchoscopy, lateral injection should be preferred because of smaller airway pressures (- 31.2 +/- 0.6%) and lung volumes, while tidal volumes remain adequate.