Circulating erythropoietin (EPO) is mainly produced by the kidneys and mediates erythrogenesis in bone marrow and nonhematopoietic cell survival. EPO is also produced in other tissues where it functions as a paracrine. Moreover, the hypoxic induction of EPO is known to be mediated by HIF-1α and HIF-2α, but it remains obscure as to which of these two mediators mainly contributes to EPO expression. Thus, we designed in vivo experiments to evaluate the contributions made by HIF-1α and HIF-2α to EPO expression. In mice exposed to mild whole body hypoxia, HIF-1α and HIF-2α were both induced in all tissues examined. However, EPO mRNA was expressed in kidney and brain, but not in liver and lung. Likewise, chromatin immunoprecipitation (CHIP) analyses demonstrated that HIF-1α or HIF-2α binding to the EPO gene increased under hypoxic conditions only in kidney and brain. A comparison of CHIP data and EPO mRNA levels suggested that, during mild hypoxia, renal EPO transcription is induced equally by HIF-1α and HIF-2α, but that brain EPO is mainly induced during hypoxia by HIF-2α. Thus, HIF-1α and HIF-2α appear to contribute to EPO expression tissue specifically.