Arginine vasopressin was measured by RIA in samples of plasma and cerebrospinal fluid (CSF) taken synchronously from 62 patients with proven or suspected disorders of the central nervous system in order to determine the relationship between the secretion of vasopressin into the systemic circulation and that into the CSF. In 12 patients without endocrine or brain disease, mean plasma values (±SD) were 2.8 ± 0.7 pg/ml and CSF values were 2.4 ± 0.7 pg/ml. Thirty-six patients with various intracranial disorders had plasma and CSF values which were both within the range of 1–4 pg/ml. Eight patients had raised plasma concentrations, but their CSF levels were within the normal range. One patient with posttraumatic diabetes insipidus and 2 patients with subarachnoid hemorrhage had concentrations of CSF vasopressin which were greater than plasma levels. These results indicate that a blood CSF barrier to vasopressin exists in man and that under certain pathological conditions excessive amounts of the hormone can be secreted into the CSF independently of that which is released into the blood, a finding which could have clinical significance in disorders of brain function.