The significance of blood viscosity alterations during anemia was evaluated in dogs under morphine-chloralose anesthesia. In group I, anemia (mean hematocrit 18.1 +/- 1.3 vol %) was produced by exchange transfusion with clinical dextran (avg mol wt 70,000). In group II, anemia was produced (mean hematocrit 19.9 +/- 0.88 vol %) with 500,000 molecular weight dextran, thus preventing the decrease in blood viscosity in group I. The cardiac output increase in group I (93.4%) with low-viscosity anemia was significantly greater than in group II (43.3%) with unchanged blood viscosity. Group III animals were transfused with a clinical dextran-red cell mixture, and group IV animals received a 500,000 mol wt dextran-red cell mixture. In group III, blood viscosity and cardiac output did not change. In group IV, blood viscosity rose and cardiac output fell significantly. The results suggest that a change in blood viscosity exerts a significant effect upon cardiac output, especially during acute dextran-exchange anemia.