1. A method has been developed which makes possible the demonstration of a pressor substance in the circulating systemic blood of dogs with experimental renal hypertension.
2. After the intravenous injection of renin into normal dogs, it was possible to detect a pressor substance formed in the systemic blood. After the intravenous injection of 1 unit of renin, as much as 1 unit of the pressor substance was detected in the plasma from 200 cc. of systemic blood.
3. Large amounts of systemic blood pooled from several normal dogs did not contain detectable amounts of pressor substance.
4. In experimental renal hypertension due to unilateral or bilateral constriction of the main renal arteries, a pressor substance was demonstrated in large amounts of systemic blood, corresponding to from one-fifth to one-third of the total blood volume. This was accomplished without the addition of hypertensinogen to enhance the action of the renin in the blood. In an animal weighing about 15 kilos, with benign hypertension up to 3 months' duration, about 3 to 5 units of this pressor substance are probably constantly circulating in the entire systemic blood.
5. The pressor substance was also detected in a relatively small amount of renal vein blood from an ischemic kidney.
6. In the systemic blood of dogs weighing about 15 kilos, with malignant experimental renal hypertension, from 15 to 25 units, or more, of the pressor substance are present in the entire circulating blood.
7. The pressor substance which appears in the systemic blood of dogs with experimental renal hypertension, and of normal dogs after intravenous injection of renin, is destroyed by hypertensinase.
8. The pressor substance obtained from the systemic blood of dogs with experimental renal hypertension has the same physiological and chemical properties as hypertensin produced in vitro. It is therefore suggested that the name hypertensin be adopted for the pressor substance which causes experimental renal hypertension.
9. In this study the animals in the benign phase of hypertension were almost all in the early stage (3 months or less). Whether the humoral mechanism obtains in animals in the late stage, after years of hypertension, or in any form of human hypertension is being investigated.