1. The intravenous injection of tyrosinase, a phenolic oxidase obtained from mushrooms, consistently lowers the blood pressure of rats made hypertensive by three different methods, while on the average not affecting the blood pressure of normal animals.
2. The intravenous and intramuscular injection of tyrosinase lowers blood pressure in dogs made hypertensive by the method of Goldblatt, while affecting that of normal ones to a less extent.
3. Tyrosinase inactivates renin, angiotonin, Victor's pressor substance, adrenalin, and tyramine in vitro, and alters the response of the blood pressure to these substances in rats and dogs.
4. Renal function of hypertensive dogs is not depressed when the blood pressure is lowered by tyrosinase.
5. Tyrosinase is an effective substance for combating experimental arterial hypertension. It is probable that some phenolic substance present in hypertension is altered.