Aspiration is a potentially fatal complication of artificial ventilation. A cuffed tube is generally used now to prevent aspiration; however, it may lead to serious complications and has several disadvantages. High frequency jet ventilation (HFJV) is an innovative technique to prevent aspiration. The trachea of 6 anesthetized, paralyzed dogs was exposed and a catheter for jet ventilation introduced between the 1st and 2nd tracheal ring. Another catheter was used for measuring intratracheal pressure. An endoscope was inserted into the trachea about 2 inches lower down and directed upwards to give a view of the vocal cords from below. A mixture of saliva, saline, and cardiogreen was introduced into the mouth so as to form a pool. When observation confirmed that HFJV prevents aspiration at frequencies of 100/min and ratios of inspiration/expiration (I:E) equalling 1:1, observations were repeated at I:E, 1:2 and 1:3 and at rates of 60/min and 200/min. The depth of the pool was gradually increased to between 2 and 31/4 inches and observations were repeated. Endoscopy alone was used in 4 animals and endoscopic film in 2 to evaluate the efficacy of HFJV. The results showed convincingly that: (1) HFJV can prevent fluid from entering the larynx from above; (2) this effect is unreliable when the frequency is decreased to 60/min or inspiration becomes shorter than 33% of the cycle; (3) intratracheal end-expiratory pressures show values slightly higher than the fluid level above the cords; and (4) the cords are separated and the gas mixture bubbles through the fluid. We conclude that (a) valve mechanisms cannot account for our observations; and (b) at rates above 60/min and with duration of expiration of 66% or less, HFJV will prevent aspiration by causing a continuous gas flow outward through the larynx. This is associated with a low continuous positive airway pressure and excellent blood gases.