1. Application of the principles of hydrodynamics to the problem of blood flow and blood volume indicates that the calculation of blood volume and cell volume from the venous hematocrit and plasma volume (as determined by the dye method) is subject to considerable error.
2. This conclusion is borne out by determinations of total cell volume by viviperfusion and with the use of radioactive iron tagged erythrocytes, which have shown the erythrocyte volume to be only 70 to 75 per cent of the volume indicated by the previously mentioned calculations.
3. The average hematocrit of the entire vascular system is considerably lower than the hematocrit of the large vessels, and the cell-plasma ratio of the smaller vessels is still less.
4. In the dog there are no considerable stores of immobilized erythrocytes, and the total erythrocyte volume and circulating erythrocyte volume are identical.
5. The "rapidly circulating blood volume" can be determined by dividing the erythrocyte volume by the venous hematocrit, and is found to be considerably less than the total blood volume.
6. The concept of the "rapidly circulating plasma volume" is introduced, and it is found to be approximately 80 per cent of the total plasma volume.
7. The volume of plasma in the peripheral, cell free, sluggishly moving plasma films, plus that contained in small vessels in which no red cells are present, is also determined and found to be approximately 20 per cent of the entire plasma volume.
8. The existence and magnitude of these fractions of the blood plasma volume should receive consideration in studies of blood flow and blood volume.