Six months residence at an altitude of 10,000 feet produced significant pulmonary hypertension and arterial oxygen desaturation (86 per cent) in 10 normal steers born at 3,600 feet. Six of these animals, during the course of the experiment, showed a rise in mean pulmonary arterial pressure from 27 to 45 mm. Hg. The remaining four animals developed more severe pulmonary hypertension, and two with mean pressures greater than 100 mm. Hg had right heart failure. All 10 showed right ventricular hypertrophy proportional to the degree of pulmonary hypertension. Ten steers of similar age and origin, maintained as controls at 5,000 feet, showed a mean pulmonary arterial pressure of 27 mm. Hg throughout the experiment. The pulmonary hypertension observed at high altitude is considered to be due to an increased pulmonary vascular resistance resulting from a reduction in the total cross-sectional area of the pulmonary vascular bed. Chronic hypoxia appeared to be the most important etilogical factor responsible for initiating the hypertension. The observed beneficial effects of 100 per cent oxygen inhalation and the prompt recovery of an affected animal when moved to lower altitude supported the concept of pulmonary hypertension induced by hypoxia.