A method has been developed for the estimation of vasopressin in blood. By means of a few simple chemical and chromatographic procedures vasopressin contained in large quantities of blood may be extracted and obtained in small volumes of solution suitable for either bioassay determinations or further purification. These methods coupled with a bioassay procedure have been employed for the study of the time course of changes in the concentration of pressor material in the blood after hemorrhage. Evidence is presented that this pressor material is identical with vasopressin. From the results of these studies, an approximate rate and total discharge of vasopressin into blood of dogs subjected to hemorrhage was calculated.