Sodium (Na) is the most important extracellular cation and the most important electrolyte in the maintenance of the osmotic pressure in cells and tissues. The endogenous losses for adult horses at maintenance are calculated to be 15 to 20 mg/kg body weight/day (Meyer et al., 1984; Schryver et al., 1987). If we assume that sodium is absorbed 100 %, then a horse with 500 kg body weight will need 7.5 grams of sodium a day. Exercise and high ambient temperatures increase sodium requirements as horses will lose substantial quantities of sodium through sweat. It is therefore difficult to calculate precise sodium requirements in a nutritional software program. Fortunately, all mammals easily excrete a surplus of sodium, as long as there is no shortage of drinking water. The most important source of sodium is common salt (NaCl).