Eleven healthy young men native to high altitude underwent haemodynamic studies at their original place ofresidence and after two years' residence at sea level. The investigation sought to determine the long-term effects ofthe suppression ofthe hypoxic stimulus upon heart rate, cardiac output, and pulmonary artery pressure of highlanders. For comparative purposes the influence of oxygen inhalation at high altitude was also studied. The heart ratefell at sea level and bradycardia was seen in most ofthe cases. Thefall in heart rate is probably related to an increase in the predominance of the parasympathetic nervous sys-tem of highlanders after the relief of hypoxia. Oxygen inhalation at high altitude produced a smaller reduction in heart rate than after descending to sea level. The increase in heart rate during exercise was similar in both environments.

Cardiac output increased moderately with the descent to sea level. This change was not associated with changes in oxygen uptake, but with a reduction in the values of haemoglobin, arterial oxygen content, and arteriovenous difference of oxygen. Stroke index increased significantly on descent to sea level, mainly as a consequence of the reduction in heart rate. At both altitudes exertion caused a similar increase in cardiac output which was related to the increase in heart rate rather than to stroke index which did not show significant variations. Oxygen inhalation at high altitude did not alter cardiac output and stroke index. Mean resting pulmonary arterialpressurefell to normal after two years' residence at sea level, but thepressure response to exercise was similar to that observed at high altitude andgreater than that seen in healthy lowlanders. Oxygen inhalation at high altitude produced only a partial reduction in pulmonary hypertension. A recurrence ofpulmonary hypertension was observed on return to high altitude. These observations emphasize the importance of the structural characteristics of t the pulmonary vessels of highlanders in the mechanism of high altitude pulmonary hypertension.