The role of the papillary muscle in left ventricular function has received new attention. We hypothesized that regional mechanics of the left ventricular wail near the anterior papillary muscle are influenced by the papillary muscle insertion. We therefore studied three-dimensional regional mechanics in and near the anterior papillary muscle in anesthetized, open- chest dogs, using implanted radiopaque markers and biplane cineradiography. In seven dogs, deformation differed little between an anterior papillary muscle insertion site (PMA) and a more basal site (PMB) overlying the anterior papillary muscle. However, local shortening and wall thickening were depressed in both locations relative to anterior free wall sites (FWA, FWB) studied in five additional dogs. A distinct structural border was observed at the junction between the myocardial wall and anterior papillary muscle, which may preclude the use of homogeneous strain in that region. Data from within the anterior papillary muscle indicated that uniaxial measurements in the papillary muscle are extremely sensitive to the orientation of the measurement axis, possibly explaining the variety of papillary muscle shortening patterns reported by previous investigators.