In open-chest dogs the circulation was carried on a heart-lung machine. The right heart and the left atrium were drained. A balloon was placed in the virtually bloodless left ventricle and filled with a known volume of air at the beginning of an experiment; left ventricular pressure was recorded after a steady state had become established. After this control observation, the preparation was exposed to experimental maneuvers, such as variation of the systemic flow rate or changes of the air volume in the balloon. At one or more later stages of the same experiment, the balloon was emptied, the heart was allowed to beat empty for several minutes, and the standard observation was again repeated. Comparison of left ventricular pressures from the initial and later observations showed that the diastolic pressure in the left ventricle increased in the course of the experiment even though other conditions were comparable. From this we conclude that experimental maneuvers injuring the heart muscle reduce its distensibility. The water content of injured hearts was abnormally elevated; myocardial edema is therefore not excluded as one of the mechanisms explaining the reduced distensibility of injured heart muscle.