Pulmonary function was studied preoperatively and postoperatively in 71 patients undergoing 41 open-heart operations, ten thoracic operations not requiring cardiopulmonary bypass, ten intra-abdominal, and ten peripheral procedures. After operation total respiratory compliance was significantly reduced below the preoperative value for ten days in the open-heart group. Decreases of lesser degree and of shorter duration were noted in the thoracic and intra-abdominal patients with no significant reduction in the peripheral cases. Tidal volume decreased proportionately to the decrease in compliance, and respiratory rate increased, while total ventilation usually remained the same or was slightly increased. Oxygen uptake was significantly increased after open-heart operations, and there was a significant negative correlation between oxygen uptake and compliance. Abnormal venous admixture, not corrected by oxygen inhalation, was observed after cardiopulmonary bypass with a return to preoperative level coinciding with that of compliance. Cardiac output did not correlate with any other measurement. The data suggest that the postoperative breathing pattern after open-heart surgery is a compensatory adjustment to decreased compliance and that reduced compliance is one of the causes of the increased work of breathing and oxygen demands after open-heart surgery.