A radioimmunoassay has been developed which permits the measurement of plasma antidiuretic hormone (arginine vasopressin (AVP), in the normal and subnormal range. Plasma was first extracted with octadecasilyl-silica to remove non-hormonal immunoreactivity, then assayed using a sensitive antibody and mono-iodinated AVP. After centrifugation of blood, we found AVP levels to be highest in the lowest layer of plasma--presumably because platelets are a reservoir for the hormone. In normal volunteers the range of plasma AVP was 0.8-14.3 pmol/l, and water loading generally lowered AVP levels. Appropriate increments in measured AVP were observed during intravenous infusion of the peptide, and during insulin-induced hypoglycaemia. Extremely low levels were found in cranial diabetes insipidus, whereas in some patients with bronchial carcinoma and hyponatraemia, AVP values were elevated. The sensitivity of the method and its practicability should make it useful both in clinical medicine and in research.