The dilution of whole blood leads to a significant improvement of its rheologic properties based on a decrease in hematocrit and, hence, blood viscosity. Under conditions of normovolemia and an adequate response of the cardiorespiratory system, the acute dilution of blood will enhance the venous return to the heart and thereby improve total and capillary blood flow significantly. In the hematocrit range of 25 to 30 per cent (limited hemodilution), this increase in flow rate is able to compensate fully for the diminished oxygen content of the blood. Changes in oxygen extraction or in oxygenhemoglobin affinity are only encountered at hematocrits below 20 per cent or if hemodilution is associated with hypovolemia. Since normovolemia is the condition sine qua non for the heart to increase its output compensatorily, intentional hemodilution should preferably be performed with colloid solutions which are capable of maintaining the colloid osmotic pressure of plasma and the circulating volume in normal limits. Limited normovolemic hemodilution with its beneficial effects on microcirculatory flow and tissue nutrition is emphasized for the treatment of impaired microcirculation as occurring in shock and low flow states, polycythemia, and high viscosity syndromes. Acute preoperative hemodilution is a means of reducing the use of bank blood and of avoiding the risks of blld transfusions in patients undergoing major elective surgery. Extreme hemodilution and total body washout in hypothermia appear to be effective clinical tools.