A rise in venous pressure caused by application of a loose binder to the arm results in a marked increase in the viscosity of the whole blood which is primarily due to a concentration of the blood in the capillaries. This concentration is shown by an increase in the viscosity and total nitrogen of the plasma, an increase in the relative volume of the red blood cells, and an increase in the relative percentage of hemoglobin. Change in the viscosity of whole blood following venous stasis apparently bears no demonstrable relation to the carbon dioxide or oxygen content.