Pulmonary compliance and shunt were evaluated preoperatively, 30 minutes after cardiopulmonary bypass, and two hours postoperatively in 132 calves undergoing open-heart surgery with halothane and oxygen anesthesia. The calves were divided into 11 groups with respect to maintenance of the lungs during bypass. In Group 1 the lungs were collapsed during bypass. In all other groups the lungs were mechanically ventilated, statically inflated, or both, with either pure oxygen or nitrous oxide, 50 per cent, in oxygen. All groups had similar compliance and shunt values before operation and sustained significant decreases in compliance and increases in shunt 30 minutes after bypass. Calves exposed to positive-pressure breathing during bypass had higher shunt and lower compliance values after bypass and postoperatively than those not exposed to mechanical ventilation, irrespective of the inflating gas or presence or absence of any amount of static airway pressure. Animals not ventilated during bypass had compliance and shunt values that were not significantly different from preoperative values, while calves that were ventilated had compliance and shunt values that were still significantly altered two hours postoperatively. These data demonstrate that positive-pressure breathing during bypass decreases pulmonary compliance after bypass and postoperatively and increases intrapulmonary shunt, but that the gas inflating the lungs during bypass does not influence either of these variables. The findings also suggest that static pulmonary inflation during bypass offers no advantage over allowing the lungs to remain collapsed.