Mammalian thermogenesis, or acceleration of metabolic rate in excess of essential metabolism, cannot be considered in isolation since it is coordinated with functions controlling heat loss to defend the body temperature. Further complexity is introduced when one considers that heat loss is effected by three discrete processes (vasoconstriction/vasodilation, sweating and changes in behaviour) and thermogenesis is also derived from three independent processes (shivering, voluntary physical activity and non-shivering thermogenesis). After recognition of a deviation in body temperature from the defended norm, heat loss is modified in a reciprocal manner to thermogenesis and, depending on age or previous training, the various sources of heat are used to achieve the necessary thermogenesis. For example, adaptation of rodents to a cold environment enhances capacity for non-shivering thermogenesis (NST) such that shivering plays a less important role in the response to cold exposure (Jansky, Bartunkova and Ziesberger, 1967).