&NA;To test the effect of different negative pressures on tracheobronchial trauma in the presence of simulated mucus, measured amounts of simulated mucus were injected into the trachea of two anesthetized mongrel dogs. Suctioning was performed using a different negative pressure for each animal. To identify the effects of negative pressure alone, endotracheal tube size, suction catheter type, suction duration, and suction procedure were identical for both dogs and were selected on the basis of current practices at the hospital with which the investigator was affiliated. Arterial pressure and electrocardiogram were monitored throughout each experiment. The trachea was excised and examined for pathologic changes. Tracheobronchial trauma occurred with suctioning at negative pressures of 100 mm Hg and 200 mm Hg; damage was greater, however, at 200 mg Hg. Results were consistent with postulates made by other investigators in that the extent of tracheobronchial trauma was directly related to the magnitude of negative pressure applied. Comparison of this study with studies which omitted mucus simulation suggests that the amount of damage is not related to the amount of mucus in the trachea. In addition, aspiration efficiency proved to be the same regardless of the negative pressure used. Suctioning at 200 mg Hg recovered approximately the same amount of mucus as suctioning at 100 mm Hg.