Both in vivo and in Hep3B cells, expression of the erythropoietin gene is induced by hypoxia as well as by certain transition metals (cobalt and nickel) and by iron chelation. When Hep3B cells were incubated in an iron deficient medium, Epo mRNA expression was enhanced 4-fold compared to Hep3B cells in iron enriched medium. The increased Epo expression in iron deficient medium was abolished when Fe2-transferrin complex was added. Epo induction by cobalt was also affected by iron concentration. In iron enriched medium, erythropoietin expression in Hep3B cells was maximally induced at CoCl2 concentrations between 100 to 200 microM. In contrast, in iron poor medium, a high level of induction was obtained at a CoCl2 concentration of only 50 microM, indicating competition between iron and cobalt. Under hyperbaric oxygen, cobalt induction of erythropoietin mRNA was modestly suppressed while nickel induction was markedly enhanced. These observations support the proposal that the oxygen sensor is a heme protein in which cobalt and nickel can substitute for iron in the porphyrin ring.