In nine anesthetized mongrel dogs anemia was produced by exchanging blood with plasma substitute thus reducing hemoglobin gradually in three steps. Aortic, atrial and ventricular blood pressures, cardiac output, electrocardiogram, phonocardiogram and the first derivative of the left ventricular pressure were continuously monitored. Blood samples were taken to determine hemoglobin, blood gases and whole blood viscosity. Progressive hemodilution resulted in a significant increase in cardiac output and left ventricular stroke work, while total peripheral resistance, oxygen content and whole blood viscosity decreased significantly. There were no significant changes in cardiac pressures, myocardial contractility, diastolic pressure time index and blood gases. The oxygen supply/demand ratio had gradually declined, while electrocardiogram showed no significant changes. These results suggest that moderate isovolemic hemodilution in animals with normal coronary vessels does not impair left ventricular function as this was manifested by the unchanged hemodynamic and electrocardiographic findings.