A thorough understanding of the present concept of corrective surgery for mitral insufficiency substantiates the author's belief that it is an urgent technical challenge to secure the patients with advanced mitral insufficiency by balancing the increasing hazard of operative risk against the decreased and improved poor risk state as exists preoperatively. For this purpose, the author studied the effect of ligature of inferior vena cava upon experimental mitral insufficiency in dogs by means of hemodynamical, electrocardiographical and radiologic methods. Fifty dogs have been used in this experimental study. The following technical and patho-physiological innovations have been clarified: 1. The experimentally created mitral insufficiency utilizing the technique to cut the papillary tendons of posterior cusps relative widely revealed very similar hemodynamical patho-physiological patterns to human patients. 2. Marked improvement in decreasing the mortality has been found as follows: the mortality of dogs created experimental mitral insufficiency was 50 % and, in contrast to this, the mortality of dogs treated by ligature of inferior vena cava at the lower level of branching portion of renal veins was only 29.6 % . On the other hand, this ligature technique has been very effective to prevent the dog from immediate postoperative death or occurrence of acute and chronic pulmonary edema. 3. The hemodynamical pattern immediately after the ligature of vena cava upon experimental mitral insufficiency showed a marked improvement to restore more physiologic hemodynamics. Left atrial, pulmonary capillary and pulmonary arterial pressures increased by the creation of mitral insufficiency decreased markedly immediately after the ligature. This improved pattern is likely to be a precise reflection of decreased overloading of right ventricular system due to the decreased venous return into the heart. 4. The long durated postoperative observation on the same research also demonstrated the similar hemodynamical improvement as mentioned in item 3. Morever, the decreased pulmonary resistance seen in the postoperative observation indicated a marked hemodynamical improvement. These improved patterns durated at least one month latter after ligature. 5. In the electrocardiographic study, it has been thought to be most feasible to devide into two major diverse types of the experimental mitral insufficiency as follows: the first is high pressure group in which the left atrial pressure increased over 10 mmHg in mean pressure. The second diverse group is low pressure group with increased left atrial pressure lower than former. The temporary electrocardiographic improvement has been found in high pressure group showing the temporal shortening of the prolonged QT-ratio by ligature of inferior vena cava. Dogs with right ventricular strain used to fail to recover in long term observation. On the other hand, the low pressure group revealed the more definite electrocardiographic improvement durated about one month latter after ligature. The improved pattern has been, for examples, that increased P(Ⅱ) and P(Ⅲ) waves showed tendency to lower and prolonged QT-ratio showed tendency to shorten. 6. In the radiologic study, the author applied the left ventriculography to demonstrate mitral regurgitation, the left atriography and the simple radiography of the heart in anteriorpoesterior position. Results indicated that ligature of inferior vena cava has the reducing effect of the size of atrial chamber and heart in general. However, this has no influence on the degree of mitral regurgitation. Thus, it has been very clear that the ligature of vena cava improves the cardiac function definitely. 7. In view of the fact that these improved patterns demonstrated in above items and since it is a clear-cut fact that the patients with advanced mitral insufficiency uniformly require the clinical improvement of their clinical signs preoperatively, it would therefore seem to be very necessary to restore the patients utilizing the ligature of inferior vena cava by which improvements in hemodynamical and cardiac functional patterns have been demonstrated clearly in experimental mitral insufficiency.