When hypoxia increases during ascending in the mountains, ventilatory control and the related oxygenation may be challenged. The pre-treatment by intermittent hypoxia will elevate ventilation and offset hypoxemia and acetazolamide may inhibit peripheral chemosensitivity and act through central mechanisms. To study these effects in the field, one well-trained male mountaineer performed four ascents from low (1300m) to higher altitude (2600m): (1) under control conditions, (2) after intermittent hypoxia, (3) after pre-treatment with acetazolamide, and (4) after intermittent hypoxia+acetazolamide. When ascending under control conditions a cascading decrease of arterial oxygen saturation (SaO 2 ) has been observed probably because of the alternating dominance of peripheral and central mechanisms of ventilatory control. While the pre-treatment with intermittent hypoxia prolonged the constant SaO 2 periods, the intake of acetazolamide eliminated this respiratory periodicity. Oxygen desaturation was best prevented by acetazolamide which was also associated with faster ascent times compared to control conditions.