Ten healthy female subjects performed maximum exercise on a bicycle in an altitude chamber during normoxia and hypobaric hypoxia simulating altitudes of 2,450, 3,700 and 4,600 m. The increases in systolic blood pressure responses were reduced with the degree of hypobaric hypoxia, whereas heart rate and diastolic pressure responses were unchanged. The increases in blood levels of aldosterone, plasma renin activity, adrenaline, noradrenaline, neuropeptide-Y and vasoactive intestinal peptide were similar at the different simulated altitudes. Angiotensin-converting enzyme and vasoactive intestinal peptide levels were not affected by hypoxia or maximum exercise. The present results suggest that the decreases in systolic blood pressure responses during hypobaric hypoxia could not be explained by altered responses of the measured vasoactive substances from the renin-angiotensin, gastrointestinal, and autonomic nervous systems.