In any situation where heat production as a result of physical exercise exceeds heat elimination from the body by radiation and convection, the body will depend on sweat secretion and evaporation for its thermoregulation. Sweat secretion will reach maximal levels at high energy expenditures in the heat but will be limited when exercising in the cold climate. Athletes and their coaches should understand some of the principles of thermoregulation in order to make an adequate decision about optimal fluid and carbohydrate replacement in a specific situation. In general it is advised that the carbohydrate content of rehydration drinks should be low (max 80 g l-1) when sweat loss is maximal, may be intermediate when both carbohydrate availability and moderate dehydration influence performance (up to 110 g l-1), and may be maximal (up to 160 g l-1) when the sweat loss is minimized and carbohydrate is the major determinant of the rate of fatigue development. Sodium should be added to rehydration drinks in order to maximize fluid and carbohydrate absorption. A range of electrolyte values for replacement of sweat induced losses, based on whole body wash down procedure is presented.