The present study addresses the controversy of whether the reduction in energy metabolism during torpor in endotherms is strictly a physical effect of temperature (Q10) or whether it involves an additional metabolic inhibition. Basal metabolic rates (BMR; measured as oxygen consumption, VO2), metabolic rates during torpor, and the corresponding body temperatures (Tb) in 68 mammalian and avian species were assembled from the literature (n = 58) or determined in the present study (n = 10). The Q10 for change in VO2 between normothermia and torpor decreased from a mean of 4.1 to 2.8 with decreasing Tb from 30 to less than 10 degrees C in hibernators (species that show prolonged torpor). In daily heterotherms (species that show shallow, daily torpor) the Q10 remained at a constant value of 2.2 as Tb decreased. In hibernators with a Tb less than 10 degrees C, the Q10 was inversely related to body mass. The increase of mass-specific metabolic rate with decreasing body mass, observed during normothermia (BMR), was not observed during torpor in hibernators and the slope relating metabolic rate and mass was almost zero. In daily heterotherms, which had a smaller Q10 than the hibernators, no inverse relationship between the Q10 and body mass was observed, and consequently the metabolic rate during torpor at the same Tb was greater than that of hibernators. These findings show that the reduction in metabolism during torpor of daily heterotherms and large hibernators can be explained largely by temperature effects, whereas a metabolic inhibition in addition to temperature effects may be used by small hibernators to reduce energy expenditure during torpor.