The marked intensification of experimental left ventricular failure by veno-arterial pumping which was seen in earlier experiments suggested changes of myocardial elasticity as a mechanism. Two experimental procedures were therefore applied here, in which the pressure in the coronary arteries and veins could be varied at will, where the left ventricle was distended by an air-filled balloon, and where the coronary tree did not communicate with the left ventricle. Changes of the coronary arterial or venous pressures were accompanied by homodirectional changes of the left ventricular diastolic pressure which were of large magnitude and which could not be explained by unobserved blood flow into the left ventricle or by other factors.The inverse relationship between coronary vascular pressures and myocardial distensibility was probably caused by the increasing volume of blood which was retained in the coronary arteries and veins when the coronary arterial or venous pressures were increased. This passive increase in coronary blood volume (turgor) must have changed the resiliency of the coronary tree. The changed elastic properties of the coronary tree then resulted in a change of the elastic properties of the heart.