A radioimmunoassay for AVP capable of measuring human plasma AVP is described. Iodination was performed by the chloramine T method and purified by chromatography on Sephadex G-25. Specific activity of 125I-AVP was 1710 +/- 155 Ci/mmol. Antiserum of high affinity (Keq = 2.7 X 10(11) 1/mol) has been raised in rabbits, which shows slight cross-reactivity with LVP and negligible reactivity with oxytocin. The aqueous assay is capable of detecting 0.4 fmol of AVP/tube and it is highly reproducible. A F lorisil extraction technique is described in detail and gives recovery of 70% of synthetic AVP added to plasma over a wide physiological range. The lowest detectable concentration of plasma AVP is 0.3 pmol/l. The method has been validated by studying changes in plasma AVP concentration following overnight dehydration (plasma AVP =3.46 +/- 1.89 (SD) pmol/l), and water loading (plasma AVP = 1.54 +/- 0.59 pmol/l), P less than 0.005, in normal subjects. A highly significant positive correlation has been found between plasma AVP and plasma osmolality (r =+0.75). Plasma AVP concentration has also been determined in patients with DI and the syndrome of inappropriate ADH secretion. No effect was found on the level of plasma AVP in normally hydrated volunteers undergoing postural change but levels rose following strenuous exercise from basal concentrations of 1.57 +/- 0.59 pmol/l to 4.77 +/- 3.43 pmol/l, P less than 0.01.