the optimal hypoxic dose for sea level performance adaptation is not a new concept. Too long and/or too extreme exposure and training/performance is compromised, whereas too short or too low is insufficient to stimulate any worthwhile physiological adaptation. Rusko et al. (32) suggested a minimum exposure for athletes of 12 h/day for at least 3 wk at altitudes 2,000-2,500 m. In a comprehensive review, Wilber et al. (43), increased the duration to 4 wk at natural altitudes of 2,000-2,500 m, whereas for simulated live high:train low (LHTL) he suggested that 12-16 h/day at higher altitudes (3,000 m) is required for accelerated erythropoiesis to occur. Contrary to these recommendations, the time course of the hemoglobin mass (Hbmass) response appears faster than conventionally accepted (8, 12). The mean response is on average ∼1% per 100 h of exposure, indicating that 2 wk of hypoxic exposure might be sufficient, at least in some cases (12), to elicit some erythropoietic adaptation.