This chapter reviews the regulatory mechanisms underlying water and sodium metabolism, the two major determinants of body fluid homeostasis and the major disorders of water metabolism, hyponatremia and hypernatremia with relevance to neurosurgery. In cases of neurological deterioration with no obvious surgical cause, electrolyte disturbances should be suspected in addition to hypoxia and hypercapnia. Hyponatremia may be due to SIADH and CSWS; CSWS is more common in acute intracerebral pathology. SIADH requires fluid restriction and CSWS requires fluid replacement; hypernatremia may, in addition to fluid replacement, require judicious use of aqueous pitressin. The correction should be slow, not exceeding 10 mEq/l in the first 24 hours; the exception is perioperative hyponatremia, often an acute process (48 hours), associated with a higher risk of developing complications.