Decreased oxygen availability in sojourners requires adjustments in tissue oxygen supply, the most effective of which is an increase in the hemoglobin (Hb) concentration. It is achieved by two independent processes: a fast increase in Hb is achieved by decreasing plasma volume due to enhanced renal Na- and water excretion. A further but slow increase in Hb concentration is achieved by stimulation of erythropoiesis by mechanisms depending on stabilization of hypoxia-inducible factor 2α resulting in elevated levels of erythropoietin in blood. The magnitude of decrease in plasma volume and of stimulation of erythropoiesis depends on the degree and duration of exposure to hypoxia at high altitude. Upon descent from high to low altitude elevated O₂-transport capacity is no longer needed. Thus, plasma volume can be restored and excess erythrocytes can be removed from circulation. This latter process is called erythrolysis. Its effectiveness seems to depend on the altitude to which individuals had been exposed. Whereas most of the excess erythrocytes seem to be removed from circulation within 1–2 weeks after a stay at altitudes > 3500 m, total Hb mass seems to remain elevated for up to 4 weeks when individuals had been exposed to more moderate altitudes, e.g. in the range of 2500 m. These are the altitudes where athletes typically perform altitude training. Thus, it appears that improved performance in the weeks after return from altitude training depends in part on maintaining elevated total Hb mass, which is known to increase aerobic capacity.