It appears to be a fact that the giving of digitalis decreases the cardiac output (Burwell, Neighbors and Regen, and Stewart) and cardiac size (Stewart) in normal human beings. The effect of its administration when hearts are diseased is, however, still unknown, With the introduction by Grollman, of the acetylene method for measuring the cardiac output in man, a method became available for studying this condition. In this investigation the method of Grollman has been used. In addition to estimations of the cardiac output, the size of the heart was measured from x-ray photographs taken at a distance of 2 meters. In addition to the usual hospital records, electrocardiograms were made and the heart rate counted. Blood pressure and vital capacity were also investigated, but not reported here. All observations were made with patients in a basal metabolic state, immediately before digitalis was given and at frequent intervals afterward. The first patient, a male, 47 years of age, suffered from a first attack of heart failure. The records secured in studying his case illustrate the course of events in cases forming the group exhibiting normal cardiac mechanism and heart failure of the congestive type. The diagnosis was generalized arteriosclerosis, hypertension, rheumatic heart disease (mitral insufficiency), cardiac hypertrophy. His heart and liver were large. Ascites and edema of the lower extremities were present. The lungs were free of rales. During heart failure the cardiac output measured 2.88 liters per minute, i. e., it was diminished, and the heart now 198.3 sq. cm. in area was larger than during the stage of compensation. On the first occasion that digitalis was given, diuresis occurred and the patient became free of the signs and symptoms of heart failure.