The hypothalamic-neurohypophyseal system functions to maintain plasma osmolality within narrow limits. It also is an important mechanism in maintaining normal body fluid volume. The system exerts its influence via release or inhibition of vasopressin (antidiuretic hormone, ADH) which acts on the kidney to decrease water excretion. Deficiency of ADH is usually due to hypothalamic-neurohypophyseal lesions (central diabetes insipidus) or insensitivity of the kidney to ADH (nephrogenic diabetes insipidus). These patients, if untreated, have the predictable result of dehydration, hyperosmolality, hypovolemia, and eventual death in severe cases. On the other hand, ADH excess of the syndrome of inappropriate ADH secretion due to a variety of causes promotes water retention, hypoosmolality and hyponatremia which, if untreated, may progress to convulsions, coma, and death. It is obviously important to diagnose accurately these pathologic states of hydration. Not only is initiation of treatment in general dependent upon recognition of the disease, but each type of pathologic hydration state has specific treatment which rewards both patient and physician with effective correction of the problem.