Today anatomical studies on the bone marrow, particularly on features of vascular construction are almost completely known, and no one can refute that more than a half of the bone marrow is composed of blood vessels, thus naturally the space occupied by the vessels is quite extensive. In view of this, with the use of an isotope I(131) as a tracer, the author has estimated in figures the volume of blood in the bone marrow of adult rabbits as well as that of the spleen whose vascular construction is similar to that of the bone marrow at the same time measured in the same manner the volume of blood in the bone marrow and the spleen of various experimental anemic rabbits. After comparing the results of those two groups, the following data have been obtained: 1) On the average, 0.5 c. c. of blood is contained in 1 g. of the bone marrow of the normal rabbit, the average of 0.44 c. c. in the spleen, and in 1 g. of muscle 0.07 c. c. of blood is containd on the average, thus it is clear that both the bone marrow and spleen are the organs rich in blood. Especially the voluminousness of the blood vessels in the bone marrow has been proven in figures from the peculiar vascular construction itself which enables an anatomical estimation. 2) The amounts of blood in the bone marrow and spleen decrease accompanying the decrease in the amount of blood in the circulation by bleeding, and following the recovery of the amount of circulating blood the recovery in the spleen is faster than that in the bone marrow. This due to the fact that the spleen is more elastic of the two organs. 3) As regards various experimental anemic rabbits, especially in those caused by leucocyte toxins (x-ray irradiation, injection of nitrogen mustard, or administration of benzol), the amount of blood in the bone marrow has been found to have decreased in the bone marrow whereas in the spleen it has been found to have increased. This phenomenon can be explained by the fact that in the bone marrow it is caused by disturbances of blood circulation while in the spleen by hyperemia. Again in the case of injection of phenylhydrazin which is considered to be especially toxic to erythrocytes, the amounts of blood both in the bone marrow und spleen do decrease but this is so because in this case the blood circulation is disturbed in both organs.