Whole-body heat production is a consequence of several biochemical events. Specific sites of heat production have been isolated, some being greater contributors to whole-body oxygen consumption than others. Tissues of the splanchnic bed (the gastrointestinal tract and liver) make up only 44% of whole-body mass, but account for 40% of total ATP utilization. Skeletal muscle accounts for a much larger proportion of whole-body mass (50%), but accounts for only 20% of whole-body ATP use. Two major biochemical events contributing to energy use are Na+, K+-ATPase (EC 3.6. 1.3) activity and protein turnover. Discussion of these two processes will dominate the present paper but reference will be made to other energy-consuming processes within selected tissues. First, a major contributor to cellular energetics is the activity of the enzyme Na+, K+-ATPase, which extrudes three Na+ from the cell and moves two K’into the cell against their respective concentration gradients at the cost of one high-energy phosphate bond (Balaban et al. 1980). The actions of this enzyme are responsible for the active transport of substrates, maintenance of ionic homeostasis, membrane potential and cell multiplication (Rosier et al. 1987; Huntington & McBride, 1988).