Humidification of inspired gases is indispensible to prevent serious injury to the tracheal mucosa of patients on mechanical ventilation. High-frequency jet ventilation (HFJV), recently introduced for the management of some forms of respiratory failure, presents unusual technical problems of humidification. The present investigation evaluated a technique to humidify jet gases by delivering small droplets of normal saline in front of the injector cannula, and utilizing the jet flow to nebulize the fluids administered. One millicurie of technetium-99m sulfur colloid (Tc-99m) was diluted in 10 ml of saline and infused in the airway of nine patients supported on HFJV. Six patients were orotracheally intubated and three were tracheostomized. A portable gamma camera was used to determine the distribution of radioactive contrast in the airway every 200 sec. When the injector catheter was directed upward, as was the case in tracheostomized patients, contrast material accumulated in the ventilator lines, and visualization of the airway was limited to the trachea. In orotracheally intubated patients, the injector catheter was generally directed downward, and radioactive contrast material extended beyond the major bronchi. The radioactive tracer reached the most distal airway location in patients with the lowest PaCO2 values. Thus, when humidification systems rely on jet-stream nebulization of fluids, the position of the injector cannula should facilitate saline flow by gravity. In the presence of poor alveolar ventilation, the concomitant risk of inadequate humidification should be considered.