Under conditions of progressive hypoxia, oxygen transport was compared in bar-headed geese (Anser indicus), a species which breeds on the Tibetan Plateau and migrates at altitudes up to 9200 m, and Pekin ducks (Anas platyrhynchos, forma domestica), a similarly sized, sea-level water fowl that does not fly. Pekin ducks showed no altitude-induced behavioral effects (e.g., restlessness) up to 7620 m, while bar-headed geese tolerated 10,668 m with no observable behavioral changes. Ventilatory and cardiac responses to hypoxia as functions of PaO2 followed a typical hyperbolic contour, but the response began at almost 20 Torr lower in the bar-headed goose. Both ventilation and cardiac output appeared to follow a common response curve for the two species, when the independent variable was expressed as arterial oxygen content. The goose had a high oxygen affinity hemoglobin, compared with the duck; the oxyhemoglobin curves of both shifted slightly to the right as a result of acclimation to 5640 m; but only the duck developed erythrocytosis as a consequence of acclimation. Under sea level conditions the duck maintained a higher mixed venous PO2, but with acute hypoxic exposures PVO2 was higher in the goose. Following acclimation, cardiac output in the duck was lower than in pre-acclimatized state, but in the goose it was higher up to the altitude at which it migrates. The selective pressures leading to the evolution of favorable oxygen transport in the bar-headed goose are discussed.