It seems established that arterial hypertension must be due to increased peripheral resistance, since the other factors which are involved in the maintenance of blood pressure—cardiac output,1 volume of the blood2 and viscosity of the blood3—have been proved to be normal in patients with hypertension. Since it has recently been shown that the blood flow in the arm is normal in hypertensive patients,4 it is probable that the increase in resistance is widespread, affecting all parts of the body and not merely, as had been suggested, the splanchnic or other localized areas. Moreover the increased resistance must be due to hypertonicity of the vessels, i. e., a functional constriction, not due to a structural narrowing, since relaxation of these hypertensive vessels can be obtained.4a The evidence concerning the exact cause of the increased peripheral resistance is still somewhat negative, since it has been shown not to be of