SummaryExposure to a 50% oxygen environment produced a decrease in erythropoiesis which was paralleled by a decrease in plasma erythropoietic stimulating activity. Similar results were obtained following exposure to a 100% oxygen environment. Marrow depression produced by hyperoxia could be reversed by erythropoietic stimulants. The suppression of erythropoiesis in both partners of parabiots maintained so that one breathed 20% and the other 90-95% oxygen suggests that a humoral inhibitor is produced in the hyperoxic animal and supports the hypothesis that erythropoiesis is regulated by the dynamic interaction of erythropoietin and an erythropoietic inhibitor.