Right heart catheterization studies were performed in 38 healthy men 17 to 34 years of age born and living at high altitudes. In order to obtain comparative data, a similar investigation was made in 25 healthy men born and living at sea level. Previously, a physical examination was performed, and roentgenograms of the chest, hematological, electrocardiographic and vectorcardiographic data were obtained. Mild pulmonary hypertension and a moderate increase of the pulmonary vascular resistance and right ventricular work were found in men living permanently at high altitudes. Pulmonary wedge pressure, cardiac output and heart rate did not show significant differences from the data obtained at sea level. The changes occurring in the heart and pulmonary circulation in men living permanently at high altitudes are not quite comparable with the changes described in temporary residents at high altitudes, nor with those experimentally obtained by acute hypoxia. This means that when studying the effects of hypoxia upon the heart and pulmonary circulation, it is important to bear in mind not only the degree of hypoxia but also the time of exposure to it. The augmented pulmonary vascular resistance in the high altitude dweller is related to the anatomic changes in the small pulmonary arteries and arterioles which have been described by other investigators. Functional factors such as vasoconstriction, hypervolemia and polycythemia do not play an important role in the mechanism of the pulmonary hypertension at high altitudes. The role of the pulmonary hypertension in the complex mechanism of acclimatization to life at high altitude is not well understood. Apparently pulmonary hypertension would not accomplish a useful part in this mechanism. It is possible, however, that pulmonary hypetension, in association with other factors such as hyperventilation and an extensive capillary bed of the lungs, does play a role in improving the arterial oxygenation in men living at high altitude. © 1963.