The use of chronic indwelling cannulas has permitted blood gas and pH determinations in unanesthetized euthermic (37 C) ground squirrels (Citellus tridecemlineatus) as well as in undisturbed hibernating (5 C) ground squirrels. The arterial pH of the euthermic animals is 7.385 [plus or minus] 0.060 and of hibernating animals is 7.390 [plus or minus] 0.030. The arterial carbon dioxide pressure [PCO2] in the euthermic ground squirrels is 42 mm Hg and in the hibernating animals is 35 mm Hg. The similarity of these values suggests that the arterial pH and PCO2 are kept constant over a wide range of temperatures. This constant PcO2 necessitates a change in total CO2 content from 26 mmoles/liter at 37 C to 38 mmoles/liter at 5 C. Constant values for arterial pH and PCO2 also were demonstrated during arousal from hibernation in which the body temperature changes rapidly. The in vitro titration of blood demonstrates that at a constant PCO2, the pH changes very little with changes in temperature (ground squirrel blood 0.0009 pH unit/[degree] C). Thus, a plausable mechanism for maintaining a constant arterial pH over large temperature changes is the respiratory control of the PCO2.