Twenty -five young male guinea pigs were placed in a decompression chamber, at a simulated altitude of 14,000 feet, for periods ranging from 1-5 days, at the end of which quantitative studies were made of cellular changes in marrow and blood and compared with similar data obtained from 33 controls. During the 5 days of hypoxia, the red cells in the blood roase from 4,700,000 in control animals to 5,500,000 per mm , while the reticulocytes rose from 59,000 mm3 to 336,000 on the third day of hypoxia, and 257,000 on the 5th. In the bone marrow, the nucleated red cells rose from 407,000 per mm3 in the control animals to 895,000 by the 2nd day, and 658,000 by the 5th day of hypoxia. The marrow reticulocytes showed little change. The marrow lymphocytes fell from 651,000 per mm3 in the control animals to 382,000 per mm3 by the 5th day of hypoxia. There were no significant changes in the marrow granulocytes.