synopsis. The physiological, morphological, and biochemical characteristics of several species of mammals resident at high altitude are compared with those of their sea level counterparts. The differences noted in these characteristics are in a direction that facilitates the acclimatization of those living at high altitude. The differences among species point to the fact that the mechanism of adaptation to altitude (i.e., hypoxia) is still not understood. This review emphasizes that the adaptive process is complex and made up of several components, that these components are inter? related, and that neither the physiological nor morphological adaptations can fully account for the tolerance to hypoxia. Although only superficially studied as yet, the biochemical adaptations appear most important.