Experiments have been carried out to evaluate the effect of continual hypoxia exposure on the hematopoietic system of mice. Three parameters have been investigated: (1) Sensitivity to x-irradiation at exposure levels between 675 and 775 R; (2) formation of endogenous erythroid colonies in the spleen after x-irradiation; and (3) response to erythropoietin injection. The results indicate that sensitivity to x-irradiation is lessened, colony formation is greater, and erythropoietin response is enhanced when these tests are performed 3 days after cessation of hypoxia on mice previously exposed for 3 to 4 weeks to air at one-half atmosphere of pressure. The findings suggest that prolonged stimulation of the erythroid system results in an enlargement of those hematopoietic compartments associated with reproduction and with response to erythropoietic stimulation.

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