Dehydration, biochemically characterized by increased concentrations of sodium and chloride in the extracellular fluid, is a frequent result of diarrhea in infants. The genesis of the hypernatremia and hyperchloremia lies in the relatively greater expenditure of water than electrolyte via skin, lungs, stool and urine. The significance of these events in terms of distribution of body water and cation are discussed. The water deficit in these infants is primarily intracellular. Cation deficits are minimal. The majority of infants with this type of dehydration present certain clinical signs that should serve to make the clinician aware of the abnormality of concentration. These infants show varying degrees of depression of central nervous system varying from lethargy to coma. Convulsions are frequently observed during the course of the disturbance. While dehydration, as measured by water retention during recovery, is invariably present, the traditional signs of water deficit are deceptively absent. Dilute solutions of electrolyte are indicated in repair. Rapid adjustment, however, of the concentration abnormality appears to accentuate the central nervous system disturbance. Repair is best carried out in a leisurely manner over a 2- to 3-day period.