We studied the dynamics of erythropoietin content and erythropoietic activity of the serum during hypoxia of different genesis and severity. Our results show that humoral factors play an important role in the regulation of erythropoiesis during oxygen deficiency. Serum erythropoietic activity underwent similar changes in different types of hypoxia associated with similar hematological shifts. A discrepancy was observed between erythropoietic activity and serum erythropoietin concentration.