Erythropoietin (EPO) mRNA levels were measured by ribonuclease (RNase) protection in organs from unstimulated rats and from animals after normobaric hypoxia or hemorrhagic anemia. Both liver and kidney responded to stimulation with large increases in EPO mRNA, but the response characteristic to graded stimulation was different. The liver responded poorly to mild normobaric hypoxia, accounting for only 2 +/- 1% of total EPO mRNA at 11% O2, but hepatic EPO mRNA levels increased steeply with more severe hypoxia so that at 7.5% O2 the liver contributed to 33 +/- 7% of the total. After hemorrhagic anemia, the liver also responded more strongly to more severe stimulation, but at all points it accounted for a significant proportion of total EPO mRNA, contributing 18 +/- 6% after removal of 2.5 ml (hematocrit 37.2 +/- 1.3%), increasing to 37 +/- 14% after venesection of 10.5 ml (hematocrit 15.8 +/- 0.8%). Studies of EPO mRNA in other organs confirmed that EPO production outside the liver and kidney were quantitatively insignificant in stimulated animals. However, the hypoxia-induced increases in EPO mRNA in brain, testis, and spleen suggest the existence of an oxygen-sensing mechanism at other sites.