Two series of patients were studied by serial measurements of blood gas exchange and pulmonarmonary dysfunction and to evaluate the dangers of respiratory failure in post traumatic patients. There were 27 patients who had sustained profound hemorrhagic shock and massive blood replacement averaging 9.7 liters and 38 patients who suffered general peritonitis or other forms of fulminating nonthoracic sepsis. All were supported by endotrachael intubation and volume controlled ventilators. The overall mortality for the post shock patients without sepsis was 12% while in the septic patients it was 35%. The maximal pulmonary arteriovenous shunt encountered in the post hemorrhagic shock patients at 36 hours averaged 20 plus or minus 8% and was accompanied by high cardiac indices (average 5.1 plus or minus 1.3 L/M-2/min) but no significant rise of pulmonary arterial pressure or peak inspiratory pressure (PIP). Severe pulmonary dysfunction subsequently occurred only in those patients who later became septic. The studies on the septic patients were divided according to the magnitude of the cardiac indices (the high indices averaged 4.8 plus or minus 1.6L/M-2/min) and thelow indices averaged 1.9 plus or minus 1.0 L/M-2/min. In the former, the average maximal shunt of 30 plus or minus 6% was sustained for 4 or more days, accompanied by an elevation of PIP to 36 plus or minus 6 cm H2O and by Pa pressure of 28 plus or minus 5 mm Hg. The patients in low output septic shock usually had an associated bronchopneumonia and had an average venous admixture of 34 plus or minus 8% and PIP values of 41 plus or minus 8 cm H2O. The mean Pa pressure in this group was 29 plus or minus 6 mm Hg.