SUMMARY: Antibodies to [8-lysine]-vasopressin (LVP) were produced in rabbits by a macrophage harvesting technique. Using these antibodies a radioimmunoassay for vasopressin was established using LVP as standard and for labelling with 125I. The antibody demonstrated complete cross-reactivity to [8-arginine]- and [8-ornithine]-vasopressin but concentrations of oxytocin 1000 times greater caused no displacement. A simple extraction procedure for antidiuretic hormone (ADH) in plasma is described. Employing this extraction procedure followed by radioimmunoassay, plasma levels of ADH were measured in sheep. The mean level of plasma ADH in nine sheep was found to be 5·28 ± 1·18 in arterial blood and this rose to 21·04 ± 7·29 μu./ml after haemorrhage of 10 to 20% of their blood volume.