Mitral regurgitation (MR) reportedly develops by ischemia of the papillary muscles, which is called papillary muscle dysfunction. This report deals with the roles of papillary muscles and left ventricular walls on the pathogenesis of MR using graded injuries of these structures in 23 dogs. Implanted ultrasonic microcrystal and occluder with an electromagnetic flowmetry for the left circumflex coronary artery were the main experimental setting. Graded occlusion of the artery was done by the six-step approach regarding coronary blood flow (CBF) reduction (C1-C6). Left ventricular (LV) pressure, systolic thickening (%W: sonomicrometry) of the LV anterior (AW) and posterior walls (PW), and systolic longitudinal shortening (%S: sonomicrometry) of both the anterior and posterior papillary muscles (PPM) were measured. MR was assessed by left ventricular contrast two-dimensional echocardiography. In eight dogs, all the data were adequate for analysis. In category 3 (C3: 55-70% CBF of control), %S in PPM decreased, but %W did not change significantly, and only mild MR developed in three of the eight dogs. MR clearly developed in category 4 (C4: 40-54% CBF as compared with the control stage), where %S was replaced by holosystolic lengthening and %W reduced to 50% of the control state, and total occlusion (C6) accompanied by significant thinning of both the PW and AW. Thus, the asynergy of the LVPW was needed to induce the MR in seven of the eight dogs. It was concluded that the injury of the PPM alone is not sufficient to cause MR, and the associated ischemic changes of the LV free wall as well as LV dilatation are necessary to induce severe MR.