SUMMARY Erythropoietin (EPO) is a glycoprotein hormone that regulates the proliferation and differentiation of erythroid progenitor cells in mammals. Although EPO has been identified in fish, the specific function and effects of hypoxia have not been investigated previously. In this study, we have demonstrated a relationship between increases in renal EPO levels and decreases in spleen EPO levels and spleen-somatic index (SSI), with increases in haemoglobin (Hb) concentration in the blood during hypoxia exposure in rainbow trout. Splenic contraction and the subsequent red blood cell release accounts for the initial increase in Hb concentration in the blood, whereas EPO action probably accounts for the later increases in hemoglobin concentration in the blood. Our data indicate that fish and mammalian erythropoietic systems are similar in response to hypoxia, in that erythropoiesis in fish is influenced by EPO.