Differential lung ventilation (DLV) may be necessary in the supportive care of patients with severe unilateral or asymmetrical lung disease. The best method for allocating tidal volume between the 2 lungs is unknown. Eighteen mongrel dogs were given a unilateral hydrochloric acid (HCI) aspiration injury. A computer-controlled differential lung ventilation system was used to ventilate 6 dogs with equal volumes in each lung, 6 dogs with equal end-inspiratory pressure in each lung, and 6 dogs with equal end-tidal CO2fraction in each lung. The computer allocated volume between the 2 lungs by these 3 schemes on a breath-by-breath basis and maintained Paco2at 35 torr and total tidal volume at 15 ml/kg. Oxygen tension fell and pulmonary venous admixture increased after aspiration. There were no differences in gas exchange among the groups. Without PEEP, differential partitioning of tidal volume is of little consequence in differential lung ventilation of asymmetrical injuries.