Although much is known about the effects of changes in electrolyte and water balance on serum sodium concentration, the quantitative relationships between body composition and the concentration of sodium in serum have not been established. Also, no definitive study has been made of the correspondence between serum sodium concentration and total serum osmolarity. It is necessary that these relationships be defined to permit classification and interpretation of abnormalities of serum sodium concentration in clinical states, as well as to extend present understanding of the effects of electrolyte equilibria and acid-base disturbances on serum sodium concentration.