Many avian species exhibit an extraordinary ability to exercise under hypoxic condition compared with mammals, and more efficient pulmonary O(2) transport has been hypothesized to contribute to this avian advantage. We studied six emus (Dromaius novaehollandaie, 4-6 mo old, 25-40 kg) at rest and during treadmill exercise in normoxia and hypoxia (inspired O(2) fraction approximately 0.13). The multiple inert gas elimination technique was used to measure ventilation-perfusion (V/Q) distribution of the lung and calculate cardiac output and parabronchial ventilation. In both normoxia and hypoxia, exercise increased arterial Po(2) and decreased arterial Pco(2), reflecting hyperventilation, whereas pH remained unchanged. The V/Q distribution was unimodal, with a log standard deviation of perfusion distribution = 0.60 +/- 0.06 at rest; this did not change significantly with either exercise or hypoxia. Intrapulmonary shunt was <1% of the cardiac output in all conditions. CO(2) elimination was enhanced by hypoxia and exercise, but O(2) exchange was not affected by exercise in normoxia or hypoxia. The stability of V/Q matching under conditions of hypoxia and exercise may be advantageous for birds flying at altitude.