Perioperative hemodynamic and temperature changes were reviewed in 58 patients who underwent aortocoronary bypass grafting. Core temperature showed an immediate decline postoperatively, secondary to core temperature cooling during bypass. Subsequent rewarming occurred over the next 8 to 12 hours, with the temperature often increasing above normal. The reason for this pyrexial response is discussed. The cardiac index was depressed immediately postoperatively, again with substantial recovery within 8 hours. This improvement over time occurred not only because of recovery of intrinsic function but also because of reduction in myocardial work due to falling systemic vascular resistance. The latter was high immediately postoperatively and then consistently fell during the rewarming phase. During the first 8 hours postoperatively there were significant changes in temperature and cardiac and systemic vascular resistance indices. The hemodynamic data correlated strongly with changes in temperature. Falling systemic vascular resistance required the institution of alpha-agonist therapy in 25% of patients.