A sensitive double-antibody radioimmunoassay for the measurement of arginine vasopressin (AVP) in plasma is described. Vasopressin has been extracted from plasma by adsorption to Florisil followed by elution with acidacetone. Assay of serial dilutions of such extracts shows that they behave in the assay system in the same way as synthetic AVP standards. Recovery of added vasopressin from plasma averaged 45.7±8.1 % (sd). Plasma AVP concentration (uncorrected for recovery) in mildly dehydrated subjects ranged from less than 1 to 4.3 pg/ml, and showed a good correlation with plasma osmolality. After nicotine, concentrations up to 21.8 pg/ml were found, and in 4 subjects with the syndrome of inappropriate ADH secretion levels ranged from 24.0 to 102.1 pg/ml.