Salient features of acute respiratory failure are reduction of functional residual capacity, decreasedlung compliance and increased right to left shunt; and pneumonia, pulmonary congestion, atelectasis pulmonary edema and fibrosis are revealed clinically. PEEP is able to prevent alveolar collapse and avoid atelectatic change and increase functivnal residual capacity and lung compliance with increment of arterial oxygen content. Decreased cardiac output and pulmonary parenchymal damages were noted during high PEEP. Blood gases were observed with IL 213 Blood Gas Analyzer during 5cm H₂O of PEEP and general anesthesia in 10 healthy persons. Following results were obtained: 1) 5 cm H₂O of PEEP increased PaO₂ from 221 torr to 275 torr and PaCO₂ from 38. 8 torr to 42. 1 torr. 2) 5 cm H₂O of PEEP increased PvO₂ from 40 torr to 48 torr and PvCO₂ from 44. 1 torr to 49. 3 torr. 3) 5 cm H₂O of PEEP increased mean arterial pressure but had no effect on heart rate. 4) 5 cm H₂O of PEEP increased C(a-v)DO₂ from 5. 306 cc/100ml to 5. 433 cc/100ml. 5) It is noted that 5 cm H₂O of PEEP is safe in healthy persons.