The present study was undertaken to obtain information concerning the cause of dyspnea in cardiac disease, with especial reference to the possibility of changes in the nature of the stimulus to respiration. Recent progress in our knowledge of the physiology of respiration and of its exciting stimulus, together with the development of certain physiological and chemical methods, have opened new pathways for the study of pathologic conditions. The pioneer work of Haldane and Priestley seemed to establish the fact that the exciting stimulus to the respiratory center is the carbon dioxid content of the blood. This conception, however, has been broadened by such subsequent investigators as Winterstein1 and Hasselbalch,2 who have shown that the essential stimulus is the hydrogen-ion concentration of the blood. The variations in the carbon dioxid content of the blood, and consequently of the alveolar air, are but the result of the