Authors deal in detail with the pathophysiology of the osmolal regulation. Besides hyperosmolality the secretion of antidiuretic hormone (ADH) in increased by hypovolemia and hypotension. Secretion of ADH is lowered in hypoosmolal states. All other mechanisms are preferebly volume regulating and they influence mainly retention and excretion of sodium. Authors discuss homeostatic effects of the renin-angiotensin-aldosteron system, effects of renal failure with prevailing glomerular or tubular function disorder, impact of diuretics, natriuretic peptides, digitalis-like hormone, urodilantin and influence of the other solutes. Disorders of the effective osmolality regulation are frequent in the cerebral affections that originate from trauma, vascular disease, inflammation or tumors. Hypoosmolality and hyponatremia are presented in two different conditions: Inappropriate Vasopressin Secretion Syndrome (IADHS) and Cerebral Salt Wasting Syndrome (CSWS). Quick differential diagnose is important because the treatment of both syndromes is essentially different. Typical cause of hypernatremia is central diabetes insipidus (DI). The group of available calculated renal function parameters is applied in the differential diagnosis of these syndromes. They are creatinin clearance, excretion fraction of water and sodium, electrolyte clearance and electrolyte free water clearance. Investigation of ADH and natriuretic peptide could be even misleading. Pathophysiologic consequence of the state given by inappropriate elevation of one hormone can be the elevation of the second one.