The present study used untrained subjects to examine the effect of acute hypobaric exposure during endurance training on subsequent exercise performance at sea level. Two groups, each of nine subjects, completed 5 weeks of endurance training [cycle ergometer exercise for 45 min, three times per week at a heart rate corresponding to 70% of that achieved at the maximal O2 consumption (VO2max) either at sea level or at high altitude] in a hypobaric chamber, under either normobaric [sea level, SL; 750 mmHg (100 kPa) approximately 90 m] or hypobaric [altitude, ALT; 554 mmHg (73.4 kPa) approximately 2500 m] conditions and the changes in SL VO2max, SL endurance time and peak blood lactate during the endurance test compared. While each group showed increases in both SL VO2max (approximately 12%) and SL endurance time (approximately 71%), there were no significant differences between the groups [SL VO2max, mean (SE)-SL group: pre-training = 42.4 (3.5), post-training = 46.1 (3.5) ml.kg-1.min-1, P < 0.005; ALT group: pre-training = 40.8 (2.6), post-training = 47.2 (3.4) ml.kg-1.min-1, P < 0.01; SL endurance time-SL group: pre-training 7.1 (1.5), post-training 11.8 (2.9) min, P < 0.01; ALT group: pre-training = 7.5 (0.6), post-training = 13.3 (1.4) min, P < 0.001]. Peak blood lactate during the endurance test was not altered by either training regimen. It is concluded that acute exposure of untrained subjects to hypobaric hypoxia during endurance training has no synergistic effect on the degree of improvement in either SL VO2max or endurance time.