A highly sensitive specific radioimmunoassay (RIA) method for the measurement of arginine vasopressin (AVP) is described. The sensitivity of the assay is 0.1 pg/ml. A simple extraction technique from plasma is described with a consistent recovery of about 65–70%, and without alteration in the immunoreactivity of the extracted hormone.AVP levels, uncorrected for recovery, ranged between 1.3 and 8.5 pg/ml (mean 3.7 pg/ml) in humans after 12–25 hr of water deprivation, and were consistently less than 1.2 pg/ml after water loading. AVP levels were undetectable in 6 patients with diabetes insipidus. Smoking was followed by up to several hundred-fold increases in plasma AVP. Plasma AVP levels in animals also rose with dehydration, increased plasma hypertonicity and hypovolemia and fell with hydration.