The effects of carbon dioxide on the cardiovascular system, cerebral, mesenteric, and renal blood flows, and total-body oxygen consumption under surface-induced hypothermia to 24 C were evaluated in 12 dogs. In Group I (six dogs), PaCO, was allowed to decrease from 35 to 18 torr during cooling without the addition of CO2 to the inspired gas mixture. In Group II (six dogs), CO2 was added to the inspired gases to maintain Paco, 34–38 torr during cooling. Arterial blood pH increased in Group I (7.39 to 7.50), while it decreased in Group II (7.35 to 7.27). Cardiac index decreased markedly with cooling in Group II, from 3.37 to 1.18 1/min/m2, while it showed an initial increase in Group I at 34 C, followed by a decrease to 1.62 1/min/m2 at 24 C. Stroke index did not change significantly, but heart rate decreased significantly in either group, with Group II showing a greater decrease. Mean arterial pressure was significantly decreased in either group from about 120 to 80 torr, but there was no significant differences in mean arterial pressures between groups at the same hypothermic temperatures. Mean pulmonary arterial and pulmonary capillary wedge pressures were essentially unchanged in both groups. Pulmonary vascular resistance showed significantly greater increases in Group II than in Group I. Internal carotid arterial blood flow was significantly greater in Group II than in Group I, but there was no difference in renal or superior mesenteric arterial blood flows between the two groups. Total-body oxygen consumption in either group decreased from about 127 ml/min/m2 at 37 C to 41 at 24 C, and there was no significant difference between groups. These results suggest that adding CO2 to the inspired gases to maintain normal Paco2 during hypothermia may be desirable for cerebral perfusion but harmful to the cardiovascular system.