1. 1. Five hundred and forty normal children were studied. Three hundred and fifty at sea level and 190 in Morococha, 14,900 feet above sea level. The comparative study was made in 5 age groups, ranging from newborn to 14 years of age. 2. 2. In the newborn the electrical activity of the heart showed similar characteristics both at sea level and at high altitudes. Some weeks after birth, the evolution of this phenomenon with age showed definite differences between the two places studied. 3. 3. At high altitudes, an accentuated right ÂQRS deviation persists during infancy and childhood. This finding, as well as the characteristics of the QRS loop and the scalar data of the QRS complex are the consequence of two factors: (a) a marked delay in the evolutionary pattern of the ventricular activation process, and (b) an increase in the magnitude of the terminal QRS vectors. 4. 4. In high altitudes, after the first weeks or months of life, the T loop shifts to a forward position and the T wave becomes positive in the right precordial leads. These characteristics remain throughout infancy and childhood. 5. 5. Our findings indicate that in healthy children of high altitudes a moderate degree of right ventricular hypertrophy exists. This is probably related to the anatomic and functional changes that take place in the pulmonary circulation as a consequence of the mechanisms of acclimatization. 6. 6. The electrocardiographic and vectorcardiographic characteristics of healthy children living permanently at high altitudes are not comparable with those of normal children at sea level. Therefore, a high-altitude environment is an important source of electrocardiographic and vectorcardiographic variability in healthy children. © 1960.