Although Hb mass was statistically similar the baseline Hbmass levels observed in the lHtH swimmers was 0.8 g kg−1higher compared to Sl swimmers. nevertheless, in 9 of the 10 lHtH swimmers, Hbmassincreased more than 3 % from baseline. thus, the observed increase is likely to be a true physiological change. Ferritin blood concentration was stable during the altitude training camp, indicating sufficient conditions for Hbmassexpansion existed. However, the reduction in blood ferritin concentration from baseline to W2POStallows for the speculation that Hbmass could have been better maintained after the return to Slin the lHtH group. In support of our findings, the same pattern of changes in Hbmass