1. Plasma from the renal vein, femoral artery or vein of normal dogs and plasma from the femoral artery, femoral and antecubital veins of man cause little or no vasoconstriction when added in small amounts to blood from a bilaterally nephrectomized dog used as perfusing medium in an isolated rabbit's ear.

2. Plasma from the femoral vessels and antecubital vein of patients with essential hypertension, malignant hypertension, and chronic nephritic hypertension causes marked vasoconstriction under the same circumstances. The plasma of dogs made hypertensive either by constriction of the parenchyma by the scar of silk perinephritis or by constriction of the renal artery by a clamp also causes pronounced vasoconstriction.

3. Plasma from the renal vein of normal dogs produces little or no vasoconstriction, but that of hypertensive dogs elicits vasoconstriction but usually not so marked as that elicited by plasma collected from peripheral vessels. A sample of renal venous plasma from one hypertensive patient caused severe vasoconstriction, not quite so intense as that produced by the peripheral vein plasma.

4. Since renin is liberated into the renal vein in large amounts in hypertensive dogs and reacts with renin-activator to produce angiotonin and since the conditions of the experiment are such as to enhance greatly the sensitivity of the ear preparation to angiotonin, it is believed that the vasoconstriction is the result of the presence of angiotonin in the peripheral blood.

5. Since vasoconstriction occurs under the same experimental conditions with plasma from both hypertensive patients and dogs, this is considered cogent evidence in favor of the view that the chemical mediator of both is similar and is possibly angiotonin.

6. A method is presented which is believed will distinguish between plasma from patients with normal blood pressure and that from those with hypertension, and between plasma from normal dogs and that from dogs with experimental renal hypertension.

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