The concentration of arginine vasopressin (AVP) was measured in cerebrospinal fluid (CSF) by radioimmunoassay. Serial dilution curves and reversed-phase high-pressure liquid chromatography (HPLC) showed that the material measured behaved identically to authentic vasopressin. Levels of CSF AVP were reduced by 37% in Alzheimer's disease, but were normal in Huntington's disease, normal-pressure hydrocephalus, and several other neurologic disorders. On direct comparison, the CSF AVP concentration was significantly lower in Alzheimer's disease than in normal-pressure hydrocephalus. Low CSF levels of AVP may therefore assist in the identification of demented patients who are not likely to benefit from ventricular shunting.