&NA;Epicardial wall motion was measured on the left ventricular free wall in six isolated potassium‐arrested dog hearts using a biplane video technique. Significant regional variations in epicardial deformations were recorded during static ventricular filling. Epicardial stretches varied linearly with cavity volume, sometimes exceeding 20% at physiological left ventricular end‐diastolic pressures. The maximum component of epicardial stretch and the derived wall thinning increased substantially from the base to the apex on both the anterior and the posterior free walls of the left ventricle. In five hearts, the direction of greatest epicardial stretch at moderate and high filling pressures coincided closely with the local epicardial fiber direction, suggesting that the left‐handed epicardial fiber helices stretch preferentially during passive filling to maximize end‐diastolic fiber lengths. Epicardial rotation was always counterclockwise, consistent with a reduction in the pitch of the fiber helix during filling. These results suggest that, on the epicardial surface, the passive myocardium is anisotropic with respect to the local fiber direction. We suggest that the resulting torsional shear acts to minimize transmural gradients of fiber stretch. (Circulation Research1989;64:721‐733)