The responses of plasma and ventricular cerebrospinal fluid (CSF) vasopressin concentration to dehydration, postural changes, and induction of nausea were studied in 21 patients with hydrocephalus of various etiology. The 24-h dehydration test evoked a significant increase in plasma osmolality and vasopressin concentration, whereas the concentration of vasopressin in CSF was unchanged. Head-up tilt to 50 degrees for 45 min with a tilt bed resulted in a modest increase of plasma vasopressin in patients who did not develop presyncopal symptoms, but no changes were seen in CSF vasopressin. Induction of nausea by subcutaneously injected apomorphine provoked a marked (20- to 50-fold) rise in plasma vasopressin concentration within 15 min, and the plasma concentration was significantly increased above base-line values for 60-120 min. Despite the prolonged period of high plasma vasopressin concentration CSF vasopressin was not influenced by the apomorphine injection. The findings suggest that the concentration of vasopressin in the CSF is controlled by mechanisms other than the well-known osmotic and nonosmotic stimuli of vasopressin release into the blood.