Hypervolemia was produced in dogs by infusion of matched whole blood or of red cells suspended in dextran. In the dogs which had received matched whole blood, bleeding with simultaneous volume replacement with dextran produced acute hypervolemic anemia. This was associated with increased cardiac output, whereas no increase of cardiac output was seen in the same animals with a comparable amount of hypervolemia without anemia. The results suggest that anemia rather than hypervolemia was the important factor in causing an increase of cardiac output in these experiments.