Electrolytes are fundamental in maintaining osmotic pressure. To correct changes in osmotic pressure and water volume, regulatory mechanisms provided principally by the central nervous system control intake and excretion of water and sodium. Disturbances in water and sodium (osmolar) result in hypernatremia, hyponatremia, hypervolemia and hypovolemia. The etiology, symptomatology and management of these syndromes are discussed. Emphasis is particularly placed on the pathogenesis and expression of the cerebral symptoms that arise from cellular dehydration and vascular damage. Hyponatremia will be discussed in part 2.