The treatment of essential hypertension with depressor drugs has been disappointing. The use of depressor substances that are present normally in the body, such as choline, acetylcholine, histamine, adenosine and its derivatives and guanido-acetic acid, has not been extensively investigated, owing primarily to the fact that unpleasant or toxic symptoms accompany their administration. However, there are theoretical reasons for believing that one of these substances, to which the name kallikrein has been given by its discoverers, may be of value in lowering a pathologically high blood pressure. The physiologic and chemical properties of this substance have been discussed in previous publications.1 This principle is presumably elaborated by the pancreas; it circulates in the blood stream and is eliminated in the urine, where it is present as a thermolabile colloid with certain rather distinctive biologic reactions on which its identification rests. These reactions are a fall in the blood pressure,