Fiber-angle changes in tangential planes through the heart wall are described in two macaque left ventricles at four equatorial and three near-apical sites. At each site, starting at a strongly negative angle at the epicardium, the fibers swing circumferentially short of midwall and continue their progressive climb to positive angles. Angles depart from the steady climb in the subendocardial region, and beyond the endocardium a wide scatter of fiber angles persist into the free trabeculae, the profuseness of which motivated this study. An interface is described histologically, coinciding with the fiber-angle deviations in the subendocardial sections, which divides the compact region of the wall from the trabeculated region. A dual function is postulated for the mural portion of the trabeculated region of the heart wall.