Bone marrow oxygen consumption (VO2) was determined weekly in 16 Holtzman rats exposed to continuous hypobaric hypoxia (CHH) during 30 days. The results were compared with those obtained in 10 sea level control animals (SL). The VO2 expressed as ng.at.O2/min, decreased progressively with time of exposure to hypoxia. VO2 (mean±SD) was 0.0936 ± 0.0135 in SL rats. In CHH animals, it was 0.1001 ± 0.0292 after 8 days of hypoxia, 0.1030 ± 0.0206 after 16 days, 0.0594 ± 0.0148(p=0.002) after 24 days and 0.0136 ± 0.0404 (p=0.000) after 30 days. Protein concentration in bone marrow was progressively higher in hypoxics when compared to control, with significant differences since the first week of exposure. Blood hemoglobin increased in parallel to protein concentration in the bone marrow. These findings suggest an increase in the cells of the erythroid series whose oxygen consumption is less than cells in the early stages of differentiation. The increased protein concentration is in agreement with the fact that globin mRNA appears in cells with a progressively increasing anaerobic metabolism at relatively late stages of erythropoiesis. © 1994.