The effects of a continuous, slow, intravenous infusion of gum acacia solution have been measured in anesthetized dogs. When the volume of the circulation was increased and the hematocrit value reduced, the cardiac output, intracardiac pressures, and peripheral blood flows began to increase. In some experiments these changes continued until sudden cardiac decompensation occurred during which the arterial blood pressure, cardiac output, and peripheral blood flows were reduced while the mean right auricular and ventricular pressures increased markedly. Analysis of the results indicates that up to the time of the highest cardiac output there is a closer correlation between cardiac output and hematocrit value than between cardiac output and mean right auricular pressure.