The development of sensitive radioimmunoassays has permitted measurement of the low concentration of vasopressin in the human cerebrospinal fluid. There is accumulating evidence to suggest that vasopressin is involved in a variety of brain functions. As an effective blood-cerebrospinal fluid barrier to vasopressin has been demonstrated, the concentration of vasopressin in the cerebrospinal fluid probably reflects the release of vasopressin within the brain. In human subjects without intracranial disease, the concentration of vasopressin in the cerebrospinal fluid is in the range 0.5-2.0 pg/ml with only little diurnal variation. Intracranial disorders associated with increased intracranial pressure may cause increased cerebrospinal fluid vasopressin concentrations, whereas degenerative brain diseases are associated with low concentrations. Only little is known about the physiologic stimuli which alter the concentration of vasopressin in cerebrospinal fluid. The concentration in cerebrospinal fluid is not influenced by a number of stimuli that cause release of vasopressin into the blood, i.e. changes in plasma osmolality, postural changes, and nausea. Elevation of the intracranial pressure, changes in the composition of the cerebrospinal fluid, electrical stimulation of the hypothalamus, and severe hemorrhage provoke an increase in cerebrospinal fluid vasopressin level.