Intracellular and extracellular acid-base parameters during normothermy and daily torpor were examined in deer mice (Peromyscus maniculatus). [14C]Dimethyloxazolidinedione and [3H]inulin were used to assess intracellular pH in liver, heart, skeletal muscle, and brain. Buffering capacities were determined using tissue homogenates. A significant increase in plasma PCO2 and CCO2 during daily torpor indicates a respiratory acidosis. All tissues experienced a reduction in the calculated dissociation ratio of histidine imidazole groups (alpha imid) during daily torpor (16.5% for brain, approximately 10% for other tissues). Based on comparisons with physicochemical tissue buffering capacities, metabolic compensation of the respiratory acidosis occurred in liver, heart, and plasma, while brain was more acidotic than predicted. The more extensive change in brain alpha imid might influence a regulated decrease in body temperature. Comparison of acid-base parameters during daily torpor and hibernation suggests that the magnitude of acid-base modifications in mammals may be associated with the level of dormancy.