The indicator-dilution method of determining cardiac output and mean transit times can be employed to estimate indirectly the volume of blood in the larger veins in vivo although it is difficult to define precisely the peripheral anatomic boundaries of the volume so measured. In 12 anesthetized dogs the “venous” blood volume obtained by multiplying cardiac output and mean transit time from femoral vein to right atrium averaged 16.6 ml./Kg., or 18 per cent of total blood volume. Rapid intravenous infusion of dextran solution caused no significant change in the blood volume of the large veins, heart or lungs.