Chronic hypoxemia was induced in one partner of a pair of parabiotic rats by exposure to defined gas mixtures with low oxygen content. The other partner of the pair was kept in normal atmosphere throughout the experiment and showed normal values of the oxygen saturation of the blood. The erythropoiesis, estimated by the percentage of nucleated red cells in the bone marrow, showed a statistically significant stimulation in both animals. It is, therefore, concluded that under these experimental conditions, the stimulus is not the partial pressure of oxygen in the bone marrow directly but a humoral factor elicited by the hypoxemia in the one partner and transferred to the other one.

ACKNOWLEDGMENTS The author is greatly indebted to Doctor U. C. Luft (Department of Physiology) for performing numerous gas and blood-gas analyses and to Mr. Allyn W. Kimball (Department of Biometrics) for the statistical analysis of the results.