Abstract 1. (1) A useful method for perfusing a dog's tail is described. 2. (2) Purified renin, which produces a marked rise in arterial pressure when injected into intact animals, has little or no constrictor action on the vessels of a dog's tail perfused with Ringer-Lockeacacia solution. The addition of small amounts of whole blood, plasma, laked erythrocytes, or plasma colloids to purified renin causes intense vasoconstriction in the perfused tail. 3. (3) Partially purified extracts of kidneys contain heart-stable and dialyzable substances that cause constriction of the blood vessels of the dog's tail. 4. (4) The injection of sufficient cocaine to make the blood vessels nonresponsive to tyramine does not inhibit the action of renin-plasma mixture. 5. (5) Plasma from dogs after bilateral nephrectomy and hypophysectomy activates renin. 6. (6) Experimental hypertension produced in dogs by clamping the renal arteries or applying cellophane to the renal parenchyma appears to increase the ability of plasma to activate renin. There is no strict parallelism between the mean arterial pressure of the hypertensive animals and activator content of the blood.