Understanding the biochemical mechanisms that enable high-altitude animals to survive and function under conditions of hypoxic stress can provide important insights into the nature of physiological adaptation. Evidence from a number of high-altitude vertebrates indicates that modifications of hemoglobin function typically play a key role in mediating an adaptive response to chronic hypoxia. Because much is known about structure-function relationships of mammalian hemoglobins and their physiological role in oxygen transport, the study of hemoglobin variation in high-altitude mammals holds much promise for understanding the nature of adaptation to hypoxia from the level of blood biochemistry to the level of whole-organism physiology. In this review I 1st discuss basic biochemical principles of hemoglobin function and the nature of physiological adaptation to high-altitude hypoxia in mammals. I then discuss a case study involving a complex hemoglobin polymorphism in North American deer mice (Peromyscus maniculatus) that illustrates how integrative studies of protein function and fitness-related physiological performance can be used to obtain evolutionary insights into genetic mechanisms of adaptation.