THE determination of angiotensin by the method of Morris and Robinson1 involves precipitation of blood with ethanol, desalting with absolute methanol, precipitation with dry ether, and assay on rats', blood pressure. They reported an average of 85 per cent recovery of added angiotensin. Pressor responses equivalent to about 1.0-1.5 ng angiotensin/ml. blood were obtained by us from normotensive human peripheral venous blood, although Morris and Robinson1 reported no angiotensin in peripheral arterial blood of normotensive patients, and complete loss of angiotensin between atrial and peripheral venous blood in patients with hypertension secondary to renal ischaemia. Also pressor responses equivalent to 1.3 ng angiotensin were obtained by us from peripheral venous blood of a patient bilaterally nephrectomized 70 days before sampling, and in 24-day-old normal human blood kept in a refrigerator. Finally, recovery of tritiated angiotensin added to blood was very low. In view of these findings, the method was re-examined leading to isolation of a vasopressor substance having properties not previously described for other vasopressor agents.