BACKGROUND: Acute hypoxia tolerance has been evaluated by the time of useful consciousness, subjective symptoms, cardiovascular changes, visual deterioration, psychological performance, and so forth. But these methods are either subjective, lacking strict objectivity, or too demanding to collect sufficient data. Pulse oximeter monitoring has come to be used not only in hospitals but in the aviation environment. This noninvasive oxygen monitoring apparatus warrants a highly objective analysis to investigate acute hypoxia tolerance. METHODS: Eight healthy male volunteers (20-25 years old) were exposed three times to a simulated altitude of 25,000 ft (7,620 m) at intervals of 7 days. We monitored arterial oxygen saturation (SaO2) and pulse rate by a pulse oximeter. The duration from taking the mask off to the time of 60% SaO2 was regarded as acute hypoxia tolerance. The venous hemoglobin concentration was measured a day before exposure to hypoxia. A questionnaire on the 14 subjective hypoxic symptoms was obtained after hypobaric chamber training. RESULTS: There were neither differences of acute hypoxia tolerance, hemoglobin concentration, nor pulse rate according to the repeated exposure of hypoxia. Four symptoms of sleepiness, fatigue, flushing and thinking impairment were sustained regardless of repeated exposures to hypoxia. But the incidence of symptoms of ear pain, anxiety, abdominal pain were reduced according to the repeated hypoxia. CONCLUSION: With pulse oximeter monitoring there was no difference between the values of time of 60% SaO2 in repeated exposures to hypoxia. The incidence of some subjective symptoms was reduced according to the repeated exposures to hypoxia.