A gas washout technique was used to measure the functional residual capacity (FRC) in eight patients during anaesthesia for cardiac surgery. The patients were anaesthetized with droperidol, fentanyl and nitrous oxide, alcuronium was given and the lungs were ventilated with a volume controlled ventilator. FRC was measured at three stages before skin incision, after sternotomy but before cardiopulmonary bypass, and after closure of the sternum. The pleural cavities were intact in all patients during the operation. FRC before skin incision was 1.7 +/- 0.5 litre (mean +/- 1 SD). A 55% mean increase in volume was noted after sternotomy and placement of the sternal retractor (P less than 0.001). Mean FRC after sternal closure was 16% lower than the preincision value (P less than 0.05). Arterial Po2 was measured in 22 other patients who underwent coronary artery bypass surgery and in whom F/o2 was 0.5. Pao2 increased significantly when the sternum was opened, but decreased after cardiopulmonary bypass. There was a further significant decrease on closure of the sternum.