Fluid ingestion during exercise has the twin aims of providing a source of carbohydrate fuel to supplement the body's limited stores and of supplying water and electrolytes to replace the losses incurred by sweating. Increasing the carbohydrate content of drinks will increase the amount of fuel which can be supplied, but will tend to decrease the rate at which water can be made available; where provision of water is the first priority, the carbohydrate content of drinks will be low, thus restricting the rate at which substrate is provided. The composition of drinks to be taken will thus be influenced by the relative importance of the need to supply fuel and water, this in turn depends on the intensity and duration of the exercise task, on the ambient temperature and humidity, and on the physiological and biochemical characteristics of the individual athlete. Carbohydrate ingested during exercise appears to be readily available as a fuel for the working muscles, at least when the exercise intensity does not exceed 70 to 75% of maximum oxygen uptake. Carbohydrate-containing solutions appear to be more effective in improving performance than plain water. Water and electrolytes are lost form the body in sweat: although the composition of sweat is rather variable, it is invariably hypotonic with respect to plasma. Sweat rate is determined primarily by the metabolic rate and the environmental temperature and humidity. The sweat rate may exceed the maximum rate of gastric emptying of ingested fluids, and some degree of dehydration is commonly observed. Excessive replacement of sweat losses with plain water or fluids with a low sodium content may result in hyponatraemia. Sodium replacement is essential for postexercise rehydration. The optimum frequency, volume and composition of drinks will vary widely depending on the intensity and duration of the exercise, the environmental conditions and the physiology of the individual. The athlete must determine by trial and error the most suitable regimen.