The cardiorespiratory effects of applying an end-expiratory pressure to the airway were studied during controlled ventilation in patients undergoing open-heart surgery. Measurements of lung function were made before and after cardiopulmonary bypass, while the chest was open, and postoperatively, when the chest was closed. The application of 10 cm H2O end-expiratory pressure resulted in a significant increase in arterial Pco2 in the preperfusion and postoperative period and a significant reduction in shunt in the postperfusion period.