When guinea-pigs were taken from sea level immediately to an altitude of 4,500 or 5,000 meters they showed dyspnea, and a large percentage of them died within from three to five days. At autopsy the lungs showed severe congestion. If death did not occur within the first week, acclimation usually resulted. This adjustment was manifested by the formation of large numbers of new minute blood vessels in the lung. The immediate reaction of guinea-pigs to high altitude indicated marked sensitivity to low oxygen tension, but the capacity for final adjustment was exceptionally good, which is in contrast to the severe pulmonary changes occurring in cats living for a few months at a high altitude. The report contains much of interest. Sixteen photomicrographs of pulmonary tissue are included.