SummaryDue to the low efficiency of the metabolic process, large amounts of heat are produced when the demand for energy is high. Since body temperature must be maintained within narrow limits, the excess heat must be dissipated and the only heat loss mechanism which can cope with high rates of heat production is evaporation of sweat from the skin; during exercise in the heat, sweat rates may exceed 2 litres/hour. Total losses in events such as marathon races may be as much as 8 per cent of body weight. In addition to water loss, some depletion of electroytes will occur, though this is generally of lesser importance. The fluid loss is distributed among plasma extracellular and. intracellular water compartments in varying proportions. From the point of view of the ability to continue working, maintenance of plasma volume is the major consideration; even small decreases in plasma volume impair exercise tolerance. The ability to replace fluid losses during exercise is limited by the rates of gastric emptying and intestinal absorption, which fall far short of the rate of loss during severe exercise. Replacement of electrolyte losses can normally wait until the post‐exercise perio