Altitude is a component of the physical environment to which animals show adjustments. Oxygen pressure and temperature decrease with increasing altitudes, and to both of these factors organisms respond in a variety of ways. As temperature varies with latitude and seasons as well as with altitude, oxygen pressure becomes the more specific factor in the study of effects of high altitudes.When man first climbs to high altitudes he has many of the sensations experienced in seasickness. He longs for his native habitat, wonders if he will survive the experience, and marvels at the ability of some of his fellows to enjoy food. I n due time, whether the environment is a rolling ship or a lofty mountain, acclimatization usually occurs. The mechanism of the adaptive process in the first case is not understood; in the second case many changes which take place during adaptation are familiar but their significance is not

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Although a considerable number of studies liave been published on various phases of the metabolism of blood cells, the relative importance of fermentation and of respiration in their normal activities is still very imperfectly understood. Still more obscure is the metabolic behavior of pathological blood cells, although it m a y be of considerable significance. It is of particular interest, for example, to examinethe activities of the cells of leucemic blood in the light of the recent advances made in our knowledge of the metabolism of cancer cells.A question which at once attracts attention in this field is that of the cause of the great variations which are found in the respiration of various types of normal erythrocytes. Several years ago it was shown by one of us (1) that normal human erythrocytes, separated from leucocytes and blood platelets by appropriate methods, have a scarcely demonstrable oxygen consumption when incubated at 38°C. for some hours under sterile conditions. The utilization of oxygen was considerable, however, when large numbers of reticulocytes were present in the blood, such as are found in hemolytic jaundice or in other conditions with active blood regeneration. These findings in general were in harmony with the previous studies of Morawitz and Itami (2) upon the oxygen consumption of anemic blood, and of Warburg (3), who found, using the Barcroft-Haldane manometer, that although normal adult mammalian erythrocytes in general had a very low oxygen consumption, the respiration was much increased in the blood of * The second paper of this series will appear in a forthcoming number of young rabbits and appeared to be roughly proportional to the amount of polychromasia present. In contrast to the slight oxygen consumption of non-nucleated mammalian erythrocytes, Warburg showed that the nucleated erythrocytes of birds (geese) have a very large oxygen consumption. By the use of the technique previously described (1), but with the more accurate Van Slyke constant volume blood gas apparatus now available, it can be shown that non-nucleated adult mammalian erythrocytes have an exceedingly small, but measurable oxygen consumption when incubated at 38°C. This is accompanied by the production of carbon dioxide.The interesting question now arises as to the cause of this difference in the metabolism of mature mammalian erythrocytes, in contrast to that of the young or immature forms and that of the nucleated cells of avian blood. To state that the oxidative activities of the different types of red blood cells seem to be related in some intimate way to the presence or absence of nuclear material furnishes no satisfactory clue as to the actual mechanism involved. It is conceivable that in the course of its development some inhibiting substance is formed in the mammalian erythrocyte which almost stops the respiratory metabolism of the cell. On the other hand, it is also conceivable that in the course of development, some essential link in the respiratory mechanism is lost in the cell so that oxidations...