This chapter concerns the body fluid changes that take place during and after exercise. Water has several colligative properties that are used to the body’s advantage. The first, and perhaps foremost, is the high specific heat of water. In addition to a role in cell metabolism as a solvent, the ionic environment of the body is owed to the ability of water to dissolve and thus allow transport of various electrolytes needed to maintain a proper internal environment. To appreciate the ion species and concentrations in the various fluid compartments, the total amount of these ions in the body should be known. When heat loss by convection and radiation is insufficient to prevent an elevation in body heat content, the increase in blood temperature flowing through the anterior hypothalamus stimulates the appropriate neurons in this area to initiate sweat gland activity. There is considerable subject variation in sweat content.