The contractility of the left ventricular wall (LV) and apex of the heart has been recorded by means of intramural probes in a series of experiments on dogs. Pressure in the heart, stroke volume and ECG were measured. Arrhythmias appeared in response to epinephrinum and ephidrinum administration. The raising strength in the apex during parasystole was accompanied by simultaneous decrease in the strength of the ventricular wall, no alterations in the hemodynamics were recorded. During precordial block the raise of the stroke volume was due to the raising strength of the ventricular wall contraction, the strength of the apex contraction remaining unchanged. The strength redistribution shown might be the cause of destructive changes in the myocardium.