Abstract
1. Exposure of lambs at sea level to air containing more than 33% oxygen resulted in a small increase in summit metabolism (maximum oxygen consumption on severe cold exposure) in some lambs, and in an improvement in the ability to maintain summit metabolism in others; there was a concomitant improvement in the maintenance of cardiac output, but no change in the degree of right to left shunting.2. Hypoxia (12-13% oxygen in inspired air) reduced summit metabolism of new-born lambs by 25%; but there was no effect in lambs 6-11 days old. Hypoxia neither reduced cardiac output, nor affected the degree of right to left shunting.3. The results suggest that in very young lambs there is some limitation of summit metabolism imposed by the supply of either oxygen or substrate to thermogenic tissues.