1. 1. Electrocardiographic observations were made in subjects taken from sea level to high altitudes, where they remained from 16 to 30 days. The first changes were observed a few hours after the arrival at high altitude, and the modifications were evident some days later. The changes disappeared 1 to 2 weeks after the subjects returned to sea level. 2. 2. The following changes were observed: (a) variations of the ÂP vector, which are probably related to wandering pacemaker in the sinus node; (b) changes of the QRS complex, which are explained by variations in the cardiac position; and (c) modifications of the ventricular recovery process, which are probably related to subacute overloading of the right ventricle and to right ventricular ischemia, as a consequence of the modifications of the pulmonary circulation in an environment of anoxic anoxia. © 1958.