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Oxygen tension of the environment is regarded as one of the principal regulators of the rate of erythropoiesis. This concept is based largely on the fact that stimulation of red blood cell formation regularly occurs at high altitudes, or under conditions of decreased oxygen tension. Evidence that the converse is true, that high tensions of oxygen can decrease erythrocyte formation, is more fragmentary. A few experiments have demonstrated that animals become anemic within a few weeks when placed in atmospheres containing 60 per cent or more of oxygen at normal barometric pressure (1-3). Early attempts to demonstrate a similar depressant effect in human subjects were unsuccessful, usually because the concentrations of oxygen were not sufficiently high, or because the observations were continued for only short periods of time (4, 5). Since the human red cell normally survives in the circulation for approximately 120 days, it is evident that special technics must be used, or the exposure to high concentrations of oxygen must be continued for weeks if an unequivocal drop in the red blood cell level of a normal subject is to be produced.Reinhard and his associates (6) circumvented these difficulties by using patients with sickle cell anemia as subjects for their studies. Because the survival time of the erythrocyte in sickle cell anemia is short (7,8), and the reticulocyte level is high, it was possible to show that the continuous administration of 80 per cent oxygen by face