INTRODUCTION Until about the middle of the nineteenth century no observations as to the total blood mass in the human body were presented by either clinicians or physiologists. In 1854, Welcker1 attempted to determine the quantity of blood in the bodies of two criminals by means of a washing out method, and he established as normal the figure of 1/13 of the body weight, which has remained the standard commonly adopted up to the present time. Even today, after a considerable series of investigations of this problem by various workers, figures given for the total blood volume in man are approximate and not absolute. The results usually quoted vary from 1/10 to 1/21 of the body weight. This uncertainty regarding the actual amount of circulating blood is largely due to technical difficulties. Clinical methods for determining the blood volume now available, however, yield results consistent enough to furnish a figure