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External respiration and gas exchange were studied in healthy volunteers during a session of intermittent normobaric hypoxia (INH) consisting of three cycles of breathing alternately a hypoxic mixture (10.7% O2) for 5 min and normal air for 5 min. The ventilatory response increased in the successive cycles of hypoxia and gradually decreased during the normoxic intervals. These changes were accompanied by an increase in carbon dioxide in lung air, which was not eliminated by the increased pulmonary ventilation during the hypoxic intervals. However, the mean oxygen consumption did not change during the INH session because the ventilatory reactivity and breathing depth, as well as the efficiency of oxygen utilization, increased from cycle to cycle.