Life emerged on our planet from a fluid environment which probably contained a similar concentration of sodium and other electrolytes to that of the extracellular fluid of mammals (1). The maintenance of the volume and composition (homeostasis) of this internal fluid surrounding all the body cells is essential for survival. Malnutrition and illness are associated with alterations in the distribution and in the renal handling of water and sodium within the body (2). Thus, an understanding of water and salt physiology is essential for the proper practice of nutritional support. © 1992.