In normal subjects the polarity and contour of the T waves are relatively constant. Atypical T waves of normal polarity occur only occasionally in the tracings of normal subjects, but are common in the electrocardiograms of patients with heart disease. In many instances the behavior of these atypical T waves and the previous or subsequent clinical status of the patient indicate that the myocardium is abnormal. During carotid sinus stimulation and after long postextrasystolic diastoles, many of these atypical upright T waves become inverted. On the other hand, many inverted T waves are converted into atypical upright T waves by exertion or angina pectoris. In some cases atypical upright T waves furnish the only objective evidence that the heart is not normal. © 1949.