Daily electrocardiograms were taken on forty-five patients with lobar pneumonia, and on seven patients with bronchopneumonia. The T-wave and R-T changes were very similar to those described in acute coronary artery closure, and the P-R, R-T, and T-wave abnormalities similar to those observed in rheumatic fever. The electrocardiogram in these three diseases is therefore not specific. In eighteen patients (35 per cent) there was a definite increase in the auriculo-ventricular conduction time, ranging from 0.20 seconds to 0.24 seconds. The impaired auriculo-ventricular conduction occurred when the temperature was normal and the patient was beginning to convalesce. Atropine sulphate affected the P-R interval only to a slight degree. There were ten patients with inverted T-waves in Leads I or II and sixteen cases in all with either inverted or flat T-waves. These patients had a poor prognosis. The T-wave inversions occurred early in pneumonia when the prostration was marked. They showed a “cove-plane” or “coronary T-wave” contour, but were, however, always shallow and always transitory. No coronary artery involvement was found in the post-mortem examinations. R-T abnormalities occurred in 93 per cent of the patients with lobar pneumonia. They first appeared with a fall of temperature to normal levels. The R-T deviations were similar to those observed in acute coronary artery closure but they never progressed to an inverted T-wave. In only one case of bronchopneumonia were there R-T changes, in marked contrast to the frequency of such changes in the lobar pneumonia cases. A tachycardia was present in thirty-five patients. The more marked the tachycardia, the worse the prognosis. A simple bradycardia was found in twenty-four patients. The rate in these cases was often about 40 per minute, once it was as slow as 36 per minute. Atropine at times released the bradycardia. Large T-waves were present twenty-one times, associated usually with the occurrence of a bradycardia. A sinus arrhythmia was discovered in twenty-two individuals, and occurred in convalescence. It was sometimes present with the bradycardia, but often appeared later. Atropine had no effect on the sinus arrhythmia. Premature beats occurred three times. Auricular fibrillation was observed in two fatal cases. “Alternation” of the QRS group occurred in two patients, one of whom died. Auricular flutter was observed once. This patient survived. Transient P-wave inversions in Lead III were recorded three times, transient T-wave inversions in Lead III alone, eleven times; a change in left axis deviation, three times; changes in the QRS group, four times. The fatal cases showed marked tachycardia, a large number of T-wave inversions ( 44 per cent), and an absence of auriculo-ventricular conduction impairment, of bradycardia, of sinus arrhythmia, and of large T-waves. It is suggested that T-wave, R-T, and P-R abnormalities are due to varying degrees of myocardial involvement and hence the electrocardiogram may perhaps serve as a guide as to when to permit a patient out of bed and when to consider him cured.