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\m=.\28\m=+-\1\m=.\18in arterial blood and this rose to 21\m=.\04\m=+-\ 7\m=.\29 \ g=m\ u. / ml after haemorrhage of 10 to 20% of their blood volume.

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1. Fifteen, previously untreated, hypertensive patients were given 20 mg of pindolol, orally. The systolic and diastolic blood pressures fell significantly in 1 h; the effect was maximal 4 h after pindolol, and persisted for at least 8 h. 2. After oral administration of 20 mg of pindolol, its concentration in the plasma reached a peak in 2-3 h. At the end of 8 h, pindolol was not detectable in the plasma. 3. There was a significant relationship between the peak concentration of pindolol in plasma and the maximal change in blood pressure in fifteen previously untreated hypertensive patients. In a separate study of nine-nine hypertensive outpatients taking 15-80 mg of pindolol daily, the blood pressure responses corresponded generally to the concentration of pindolol in plasma 2-3 h after the morning dose. 4. There were no significant changes in plasma renin activity, plasma renin concentration or plasma noradrenaline concentration in the previously untreated patients taking 20 mg of pindolol. There was no relationship between initial plasma renin or noradrenaline levels and blood pressure responses to pindolol. Nor was there any significant relationship between the changes in plasma renin or noradrenaline levels and the changes in blood pressure.

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