The relationship between left ventricular end-diastolic pressure and circumference was studied in 27 open-chest dogs, utilizing a mercury-resistance gage to measure left ventricular end-diastolic circumference. In 6 experiments aortic pressure, cardiac output and heart rate were varied independently. Tachycardia above a rate critical for each heart elevated left ventricular eud-diastolic pressure for any given end-diastolic circumference. Hypothermia at a constant heart rate had a similar effect. The altered left ventricular end-diastolic pressure-end-diastolic circumference relationships resulting from tachycardia and hypothermia are believed to be related to the incomplete ventricular relaxation which occurs as the duration of diastole is encroached upon. Acute, spontaneous heart failure was accompanied by an augmented left ventricular end-diastolic circumference for any given end-diastolic pressure, an effect which may be considered to reflect an increase in myocardial extensibility. The relationship between left ventricular end-diastolic pressure and end-diastolic circumference was not modified either by changes of aortic pressure or of cardiac output. There was no constant relationship between the left ventricular end-diastolic circumference and the tension-time index. Indeed, it was possible to manipulate aortic pressure and cardiac output so that these 2 parameters moved in opposite directions. These observations are not consonant with the view that myocardial oxygen consumption is primarily dependent on end-diastolic fiber length.