When acute coronary occlusion is produced in the dog, the conventional limb leads often yield an inadequate record of the change which takes place in the action current of the heart. However, when electrodes are applied in certain positions on the wall of the chest, clearly significant electrical disturbances can be recorded which may be partly or completely missed in the tracings of the limb leads.1 The electrocardiograms obtained from limb leads and chest leads supplement one another. If both are used together, a much more complete electrocardiographic picture of the effects of experimental coronary occlusion can be obtained. These results in the dog suggested the use of chest leads in the study of acute coronary occlusion in man.2 It was found that a similar situation presented itself: Chest leads yielded information that could not be obtained from limb leads. Moreover, it became evident that in certain cases