The purpose of this study was to determine whether differences of wall thickening and fiber shortening exist between the deep and superficial layers of the left ventricular (LV) wall during systole. By use of ultrasonic crystals, wall thickness and segment-length changes were studied in seven dogs, and changes of the internal and external diameter of the LV were studied in five dogs. Radiopaque markers were used in conjunction with ventriculograms to determine the changes of the internal and external volume of the LV in five additional dogs. The subendocardial portion of the LV wall accounted for 83% of wall thickening during diastole. Segments of the subendocardium shortened 18% during systole, whereas segments of the subepicardium shortened only 10%. A 22% reduction of the internal LV diameter occurred in comparison to only a 6% reduction of the external diameter during systole. Similar observations were made in relation to LV internal and external volumes. These results indicate that the subendocardium undergoes greater dimensional changes (both thickening and shortening) during systole than does the subepicardium.