The regulation of iron absorption is only partially understood. A cause and effect relationship between the erythropoietic and the absorption rates has been postulated, but not firmly established. Experiments in polycythemic mice were undertaken in an attempt to separate the effect of hypoxia on iron absorption from that on the erythropoietic activity. The latter parameter was evaluated on the basis of per cent values of peripheral reticulocytes, nucleated red cells in red pulp of spleen, and/or RBC-59Fe incorporation. In the first series of experiments the erythroid response to an 18- or 96-hr period of hypoxia was selectively suppressed by administration of antierythropoietin (anti-Ep) serum. However, the enhancement of iron absorption induced by an equivalent hypoxic stimulus was not significantly modified by anti-Ep. In additional studies, polycythemic mice were subjected to either fractionated Ep administration or hypoxic stimuli, lasting 24, 48, or 72 hr. The stimulatory effect on erythropoiesis induced by Ep was either greater or equivalent to that of hypoxia. However, although these hypoxic stimuli induced a four- to sixfold increase in iron absorption, Ep administration evoked either little or no enhancement of iron absorption. These experiments provide evidence against concepts linking the primary regulation of iron absorption to the erythropoietic rate.