The purpose of this study was to examine the immediate post-exercise effects of acute exposure to a simulated altitude of 4,300 m on plasma concentrations of beta-endorphin (beta-EP) and associated changes of adrenocorticotropin (ACTH), and cortisol to high intensity cycle exercise (i.e., stages of 90 and 100% peak Vo2). Exercise intensities were assigned relative to peak O2 uptake both under sea level conditions and under acute hypobaric hypoxic conditions. Plasma beta-EP concentrations significantly increased from pre- to immediately post-exercise at both 90 and 100% peak Vo2 in both the sea level and acute hypobaric hypoxic trials. No associated exercise-induced changes were observed for ACTH or cortisol pre- to immediately post-exercise at either sea level or during hypoxic conditions. Exercise at acute hypobaric hypoxia elicited no significantly different responses in plasma beta-EP, ACTH, or cortisol than those elicited by the same relative exercise intensities under normobaric normoxic conditions. Additionally, no changes in the beta-EP/ACTH molar ratio for exercise or between conditions were observed. These data indicate that acute simulated high altitude exposure neither diminishes nor augments the physiological stimuli involved with high relative exercise intensity activation mechanisms of these hormones.