A high hemoglobin mass (Hbmass) is associated with a high maximum aerobic power (VO2max), however, the extent to which Hbmass is influenced by training is currently unclear. Accordingly, this study monitored changes in Hbmass and VO2max in 12 previously untrained adults (aged 18–25 years) following 40 days of regular physical activity. Hbmass and VO2max were assessed at the start and end of a 40-day physical activity program, which comprised of approximately 40 min of daily, moderate-intensity physical activity. Relative VO2max increased by 11.3%, yet there was no significant change in relative Hbmass (1.7%) and body mass (0.2%) during the 40-day period. There was a significant correlation between Hbmass and VO2max at the start of the study (r=0.58, P=0.05), but not between the change in relative VO2max and the change in relative Hbmass (r=−0.07, P=0.83). Our results support the concept of relative stability in Hbmass with approximately 1 month of moderate-intensity physical activity suggesting that Hbmass may be used for talent identification and possibly for anti-doping purposes.