Twelve clinically healthy male calves, 1-1s years old and weighing 70-160kg, were randomly divided into 2 groups of 6. In one group (group I), pulmonary lobectomy was performed by rib resection thoracotomy and in the second group (group 11) by a mid sternal approach. There was a significant fall in paO, following thoracotomy by both rib resection and mid sternal approach. The acid base status of these animals exhibited a trend towards metabolic alkalosis. The mean arterial pressure, central venous pressure and pulmonary artery pressure maintained a low profile during midsternal thoracotomy, compared to the rib resection method. The tachycardia and ECG changes were more marked in the mid-sternal thoracotomy. Lobectomy had no effect on mean arterial pressure, central venous pressure or pulmonary arterial pressure. When the animals were weaned off from the respirator after lobectomy and chest closure by both methods, the paOz decreased greatly along with an increase in paC02. Postoperative radiography revealed moderate pulmonary oedema.