Sport embraces an enormous diversity of activities and each places different demands upon the individual. The extent of fluid and electrolyte losses during exercise will obviously have a major influence on the need for replacement during exercise, and the loss will depend on the volume of sweat lost and on the composition of the sweat. A concern with pre-exercise hyperhydration is that it may result in reductions in plasma sodium concentrations before starting exercise and therefore increase the risk of dilutional hyponatremia if large volumes of fluid are consumed during subsequent exercise. Hyponatremia is further discussed below, but it should be noted that most of the reports on exercise-associated hyponatremia assess pre-exercise sodium status. While those reported cases were therefore undoubtedly associated with exercise, it is less clear that they were caused by the exercise or by behaviors specifically associated with exercise.