Cardiac catheterization studies in high altitude natives at resting conditions have demonstrated mild pulmonary hypertension, sign catheterization studies in high altitude natives at resting conditions have demonstrated mild pulmonary hypertension, significantly higher in childreii than in adults. On exercise the pulmonary pressures increase nearly twice the resting values. A greater pulmonary hypertension is also observed in some residents of the high altitude when they lose their natural acclimatisation (Chronic Mountain Siekness). On the other hand, a significant decrement in pulmonary pressures is observed in high altitude natives after two years of residence at sea level. The pulmonary hypertension and the increased pulmonary vaseular resistance observed in people born and living at high altitudes is principally ascribed to structural ehanges of the small pulmonary art "Ties and arterioles. The role of arteriolar constriction, hypervolemia and polycythemia is discussed. © 1962 S. Karger AG, Basel.