Homeothermy is the result of an evolutionary process during which every increase in oxygen supply led to a consecutive increase in metabolic rate and, thus, to a new dependence on favorable ambient conditions. In response to the food scarcity of winter months, some inhabitants of temperate zones developed an ability to hibernate which is characterized by a fully thermocontrolled reduction in body temperature down to near zero values. Hibernation thus illustrates that in homeotherms, not only the body shell is poikilothermic, but also the core temperature is more variable than often assumed. However, in contrast to clinical hypothermia, natural torpidity does not consist of a cold-induced reduction in metabolic rate, but of an endogenous metabolic reduction with subsequent lowering of body temperature. As a factor of metabolic suppression, the pH has been suspected which, in hibernators, is kept constant at 7.4 by relative hypoventilation (pH-stat) which differs from its passive shift in the poikilothermic body shell (alpha-stat). In clinical hypothermia, temperature governs the metabolic rate in that, depending on the state of thermoregulation, either a cold defense reaction with an increased metabolic rate (accidental hypothermia) or a cold-induced reduction in metabolic rate (induced hypothermia) occurs. However, as can be learned from hibernators, the lower limit of hypothermia tolerance seems to be due to a uniform minimal metabolic rate rather than to the species-specific body temperature at which this metabolic limit is reached, depending on body size and basal metabolic rate. Accordingly, in judging the sequelae of hypothermia, the degree of cooling should be given less emphasis than the resulting effects on metabolic rate.