Twenty-one patients were studied before and after mitral valve surgery. Nineteen of them had predominant stenosis, and two had predominant insufficiency. The clinical and graphic studies are reported. The comparative value of the various methods is discussed. General criteria of exclusion from surgery are listed. Coincidence between clinical-graphic diagnosis and surgical evaluation of the valvular lesion was found in the range of about 80 per cent. Possible causes of error in the surgical evaluation of mitral incompetence are discussed. The electrocardiogram was recognized as a possible guide for evaluating the decrease of right ventricular load and the increase of left ventricular load which may follow surgical intervention. The phonocardiogram revealed changes of the first sound at the apex and of the second pulmonic sound by effect of surgery. It further revealed a practically constant increase of the interval 2-O.S. and a decrease of the interval R-1 which were considered proportional to the decrease of left atrial pressure. Changes of cardiac murmurs are also described. Phonocardiography was recognized as extremely useful in the diagnosis of mitral stenosis, in the recognition of aortic defects, and in the evaluation of the results of surgery. Electrokymography was used for the study of left atrial pattern tracings. A typical pattern, entirely different from that of normal subjects, takes place in mitral regurgitation. Possible causes of error are listed. Increased accuracy of study was obtained by taking eight to nine tracings in various projections, by calibrating the tracing, and by analyzing the pattern and its relationship with the heart sounds. The plateau pattern, previously described by Luisada and Fleischner, was explained by them as the result of mitral regurgitation. While several workers confirmed this interpretation, others raised certain objections. These objections are now refuted in detail. Two different patterns are now described: the early plateau, significant of exclusive or predominant mitral insufficiency; and the late plateau, revealing predominant mitral stenosis with moderate regurgitation. The atrial and valvular dynamics of the two conditions are analyzed, and it is concluded that the two left atrial electrokymographic patterns are the result of two different left atrial pressure patterns. The modifications of left atrial pressure by effect of surgery were typically revealed by changes in the electrokymographic patterns. Correlation of the electrokymographic data with those of other graphic methods, of the x-ray, and of clinical data led to a reliable evaluation of the mitral dysfunction. While two cases revealed evidence of exclusive or extremely predominant insufficiency, no case of stenosis, unaccompanied by some degree of incompetence, was found.