In a recent article,1one of us (R. H. M.) presented the results of estimations of the "guanidine" content of the blood of 800 patients with arterial hypertension. An increase in blood "guanidine" was found in 32 per cent of hypertensive persons with normal values for nonprotein nitrogen in the blood and in a higher percentage of patients with nitrogen retention. These observations have led directly to the following study, in which we have investigated the association in animals between high values for blood "guanidine" and the blood pressure and have also studied further the properties of the substance in the blood which with certain reagents gives a color like that of guanidine derivatives and which we have referred to as "guanidine." Estimations of the blood pressure in these experiments were carried out by the palpatory method on carotid loops, a method permitting estimation only of the systolic pressure.