1. 1. Electrocardiographic and vectorcardiographic observations were obtained in 550 normal subjects, 300 at sea level and 250 in Morococha, 14,900 feet above sea level. A comparative study was made in three age groups: the age range was 15 to 60 years. 2. 2. The ventricular activation process shows significant differences between the two places studied. In adolescent and adult inhabitants of high altitudes there is a wide range in SÂQRS direction, the configuration of the QRS complex is highly variable both in the limb and precordial leads, and the two-dimensional projections of the spatial QRS loop show wide diversity. 3. 3. Five principal QRS patterns are described according to the spatial SÂQRS orientation. These patterns do not represent different types of ventricular activation. They are varieties of a peculiar activation process which exhibits two principal characteristics: a delay in the pattern of development of the QRS changes that normally occur with aging, and an increasing magnitude of the terminal QRS vectors. 4. 4. The most common pattern in adults of high altitudes shows SÂQRS in the right inferior posterior octant, an rS complex in Lead V1, and S1-Q2-Q3 or S1-S2-Q3 patterns in extremity leads. The S1-S2-S3 pattern is also observed, and it is related to predominant late QRS vectors and not to a special cardiac position. Predominantly positive QRS complexes in Lead V1 are infrequent in adults of high altitudes. The r′-V1 pattern seen in some subjects who live at high altitudes probably represents a transitional stage in the pattern of development of QRS throughout life. 5. 5. In adults at high altitudes, right ventricular preponderance is less than in children at the same altitudes, but the physiologic preponderance of the left ventricle seen at sea level does not occur even in the older adults. The moderate right ventricular hypertrophy of high altitudes is probably related to anatomic and functional changes that take place in the pulmonary circulation as a consequence of the process of acclimatization. 6. 6. The electrocardiographic and vectorcardiographic characteristics of the adolescents and adults who live permanently at high altitudes are not similar to those of normal adolescents and adults who live at sea level. Therefore, a high-altitude environment is an important cause of electrocardiographic and vectorcardiographic variability in healthy people. © 1961.