1. 1. Ten subjects were taken from Lima, at sea level, to Morococha, at an altitude of 4,540 meters (14,900 feet). The electrocardiographic changes which occurred during 1 year of residence in Morococha were studied. 2. 2. No variations of the auricular activation process were observed. 3. 3. The following observations were made in the ventricular activation process: SÂQRS vector shifted to the right and backward. In Lead aVR the late R wave became higher. In the left precordial leads the S wave became deeper. These were progressive changes of slow development, having an anatomic significance. Two factors were probably responsible for them: variations in the cardiac position, and an incipient development of right ventricular hypertrophy. 4. 4. The following changes were observed in the ventricular repolarization process: SÂT tended to shift backward without variation on the frontal plane. In the right precordial leads the T wave became negative or abnormal in shape. Elevation of the RS-T segment was also observed in the first months. All these changes decreased or disappeared in the last months. A functional significance was attributed to them: they were the early response to the anoxemia and right ventricular overloading. 5. 5. One year of residence at high altitudes was not sufficient time to determine QRS changes as important as those observed in the native residents at these altitudes. One year of residence was also insufficient time, in most cases, for the T wave changes to disappear. The incomplete acclimatization, from the electrocardiographic point of view, after 1 year of residence at high altitudes, was in agreement with what occurred in other physiologic characteristics of the same subjects. © 1957.