Introduction: . The effectiveness of physical compensation hypoxia is known to be limited above 15,000 feet. Beyond this altitude intellectual impainnent will develop progressively. On the other hand, carbon monoxide (CD) produced in cigarette smoking presents some degrees of hypemic hypoxia The purpose f this study is to compare the effects of cigarette smoking on mental task at three selected altitude. Methods: One hundred male volunteers (50 smokers, 50 nonsmokers) participated this hypoxic experiment at 18,000 feet (N=44), 25,000 feet (N=32), and 30,000 feet (N=24) respectively. The tidal breath CO was measured and expressed as HbCO % preγiously. The smokers were allowed to have one cigarette 10 minutes before hypobaric chamber flight. After ascending to the specific altitude, they removed the oxygen mask and started to circle the number (3 and its multiples) printed on a random number list for a predetennined time period. The score was expressed as the correct percent oftask (correct circles/total circles). This mental task was also conducted on ground as control. The blood oxygen saturation (SaO2%) and heart rate were monitored throughout the hypoxic exposure. Results. The scores of perfonning at high altitude were evenly lower than those at sea level. In habitual smokers, the score was observed higher at both 18,000 feet and 25,000 feet than those of nonsmokers. But this unpredictable result was not seen at 30,000 feet. Mostly, the HbCO% of subjects was negatively correlated to their SaO2%. Conclusions. The phannacological effect of nicotine may enhance the cοncentration or alertness of subjects and thence improve their psychological perfonnance at lower altitude. However, the synergism of carbon monoxide and hypoxia do produce an overwhelming risk to mental activity at higher altitude.