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The cardiopulmonary effects of two levels of positive end-expiratory pressure (PEEP) and intermittent positive-pressure breathing (IPPB) were determined in 15 patients following heart-valve surgery. Eight patients with a cardiac index (CI) greater than 2.5 L./min./sq.m. during IPPB experienced significant reductions in CI, oxygen (O2) delivery, and O2 consumption with both 3 and 8 cm. of PEEP. Oxygen reserve, however, was not affected by PEEP. In 7 patients with a CI below 2.5 L./min./sq.m. during IPPB, PEEP3 increased CI, O2 delivery, and O2 reserve, while PEEP8 produced no significant changes in CI or O2 transport. Intrapulmonary shunting, when elevated during IPPB, was usually reduced by PEEP, and progressive increases in Pao2 usually accompanied increasing amounts of PEEP. While many factors other than PEEP may affect the O2 transport system in the postoperative period, the present study lends some assurance that the low-output state following cardiac surgery need not be a deterrent to the use of PEEP.