Maintenance of fluid balance is a major concern for all athletes competing in events held in hot climates. This paper reviews recent work relating to optimisation of fluid replacement after sweat loss induced by exercising in the heat. Data are taken from studies undertaken in our laboratory. Issues investigated were drink composition, volume consumed, effects of consuming food with a drink, effects of alcohol in rehydration effectiveness, voluntary intake of fluid, and considerations for women related to the menstrual cycle. The results are presented as a series of summaries of experiments, followed by a discussion of the implications. The focus of this review is urine output after ingestion of a drink; fluid excreted in urine counteracts rehydration. Also included are data on the restoration of plasma volume losses. Ingestion of large volumes of plain water will inhibit thirst and will also promote a diuretic response. If effective rehydration is to be maintained for some hours after fluid ingestion, drinks should contain moderately high levels of sodium (perhaps as much as 50-60 mmol/l) and possibly also some potassium to replace losses in the sweat. To surmount ongoing obligatory urine losses, the volume consumed should be greater than the volume of sweat lost. Palatability of drinks is important in stimulating intake and ensuring adequate volume replacement. Where opportunities allow, the electrolytes required may be ingested as solid food consumed with a drink. There are no special concerns for women related to changes in hormone levels associated with the menstrual cycle. Ingestion of carbohydrate-electrolyte drinks in the post-exercise period restores exercise capacity more effectively than plain water. The effects on performance of an uncorrected fluid deficit should persuade all athletes to attempt to remain fully hydrated at all times, and the aim should be to start each bout of exercise in a fluid replete state. This will only be achieved if a volume of fluid in excess of the sweat loss is ingested together with sufficient electrolytes.