The changes in oxygen consumption and carbon dioxide production were measured both before and for 6 hours after a variety of cardiac operations. Core and peripheral temperatures were measured simultaneously. Oxygen consumption was initially similar to preoperative values but rose over the next 2.5 hours by an average of 31% above basal. Carbon dioxide production was initially higher than preoperative values, then declined briefly, and finally showed an average increase above basal of 29%. Core temperature rose from an initially slightly depressed level to a maximal level 4 hours postoperatively at an average rate of 0.4 degrees C/hr. It was not possible to separate cause and effect between the temperature and oxygen consumption increases, but more effective postoperative temperature control might limit metabolic demands at a critical time postoperatively.