The experiments herein reported were undertaken with the aim of creating a chronic, sclerosing lesion of the mitral valve in dogs, as the primary step in an experimental study of the surgical treatment of mitral stenosis. Cutler1 and his collaborators demonstrated that mitral insufficiency is well tolerated by previously normal dogs. Because of their inability to perform their operation on animals with stenosis, they were unable to ascertain whether or not the abrupt transformation of stenosis into stenosis with insufficiency could be similarly tolerated. Consequently, this phase of the work, which might have been done in the laboratory, had it been possible to produce experimental stenosis in animals, was carried out on patients. A large segment was excised from the mitral valve of the fourth patient operated on by Cutler,1 and also from the valve of a patient more recently operated on by Pribram2 of Berlin. Both