In the early postoperative period, oxygen consumption was close to basal (range 71–135 per cent of basal) in fourteen non-shivering patients but raised within the range 135–486 per cent in ten shivering patients. There was no correlation between oxygen consumption and arterial Po2, which ranged from 57 to 89 mm Hg in non-shivering patients and from 61 to 96 mm Hg in shivering patients. In every case, ventilation was adequate for the increased metabolic demand of shivering and the highest arterial Pco2 was 42.6 mm Hg. Two non-shivering patients had Pco2 of 46 and 52 mm Hg respectively. In two out of three shivering patients, cardiac output was adequate for demand and mixed venous oxygen content was within the normal range. In the third shivering patient, a cardiac output of 9.4 l./min was inadequate for an oxygen consumption of 1,192 ml/min and the arterial/mixed venous oxygen content difference was 12.7 ml/100 ml. This was a factor in the associated arterial hypoxaemia (65 mm Hg). In other patients arterial Po2 was mainly influenced by venous admixture which ranged from 3.5 to 13.8 per cent.