Pre-acclimatization was conducted using a new method elaborated in our laboratory, combining high intensity exercise while breathing hypoxia normobaric gas mixtures. The training consisted in a daily training during three weeks, 6 days a week, two hours a day, on bicycle ergometer. Eighteen subjects aging 22.2 +/- 1.4 years (11 males, 7 females) were matched in two similar groups: one group trained in normoxic conditions (NG) while the other group (HG) trained with a progressive decrease of the fraction of inspired oxygen (from 12.2% to 10.0%). Maximal oxygen uptake (VO2max) were measured before and after the protocol period in both hypoxic (VO2max H, FIO2 = 10.4%) and normoxic (VO2max N) conditions, for the 2 groups. Training induced a similar O2max N increase in the two groups. The ratio VO2max H/VO2max N was calculated. As expected, in NG group, this ratio decreased significantly (from 63.9 +/- 4.3 to 57.5 +/- 3.1%, p < 0.01) after the training period compared to the initial value, diminution associated with an elevation of VO2max N (from 48.4 +/- 9.0 to 52.9 +/- 9.0 ml.min-1 x kg-1, p < 0.01). Conversely, in HG group, this ratio was not significantly diminished (from 61.7 +/- 3.8 to 60.5 +/- 5.2%, NS) in spite of a similar increase of VO2max N (from 47.5 +/- 5.5 to 50.7 +/- 4.9 ml.min-1 x kg-1, p < 0.01). This does not follow the diminution of the ratio usually described when VO2max N reach higher values.(ABSTRACT TRUNCATED AT 250 WORDS)