In order to maintain normal cellular function, mammalian tissue oxygen concentrations must be tightly regulated within a narrow physiological range. The hormone erythropoietin (EPO) is essential for maintenance of tissue oxygen supply by stimulating red blood cell production and promoting their survival. In this study we compared the effects of 290 Torr atmospheric pressure on the kidney EPO protein levels in young (4-month-old) and aged (24-month-old) C57BL/6 mice. The mice were sacrificed after being anesthetized, and kidney samples were collected and processed by Western blot analysis. Relatively low basal expression of EPO during normoxia in young mice showed significant upregulation in hypoxia and stayed upregulated throughout the hypoxic period (threefold compared to normoxic control), showing a slight decline toward the third week. Whereas, a relatively higher normoxic basal EPO protein level in aged mice did not show significant increase until seventh day of hypoxia, but showed significant upregulation in prolonged hypoxia. Hence, we confirmed that there is a progressively increased accumulation of EPO during chronic hypoxia in young and aged mouse kidney, and the EPO upregulation during hypoxia showed a similarity with the pattern of increase in hematocrit, which we have reported previously.