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Measurements were made to characterize the relationship between erythropoietin output and erythropoiesis in two groups of subjects, one moved from a sea level habitat to high altitude, and the second moved from a high altitude habitat to sea level. In the first group, there was a latent period of 6 hours followed by a rapid increase in erythropoietin, and a secondary fall to a level of approximately twice normal. The increased erythropoietin stimulus was also reflected in a shortened marrow radioiron transit time. In the second group, there was an initial unexplained rise, after which erythropoietin fell within 8 hours to undetectable amounts. Elevated erythropoietin was associated in Group I with an increased iron uptake within 24 hours of the stimulus, suggesting a direct action of erythropoietin on hemoglobin synthesis by the existing marrow population. Limitation in erythropoiesis to a rate of less than twice normal was tentatively explained by a restricted iron supply. In the second group, marrow activity continued for 3 days despite a marked fall in erythropoietin, indicating that cells in the maturation phase completed their normal development.

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