The author examined Qo(2) in various regions and Q(NM2) in the diaphyseal part of the femur of normal rabbit bone marrow from viewpoint of the age range, namely, in five groups, one-month-, two-month-, three-month-, six-month-, and one-year old rabbits. 1. In the normal rabbit bone marrow, significant differences in Qo(2) were found according to the age range. 2. Qo(2) of the one-month group had a wide range of Qo(2), two-month and three-month groups had a narrower range, six-month and one-year gruops had a wide range of Qo(2), showing an especially lower value in the radius. 3. When observed from the morphological point of view, Qo(2) of the one-month group showed higher values in all regions and the decrease of Qo(2) in all regions from the one-month group to the three-month group is parallel with the decreas e of erythroblast percentage; but in the six-month group despite a greater decrease of erythroblast percentage Qo(2) except that in the radius shows a slight increase; and in the one-year group apart from a marked increase of erythroblast percentage all Qo(2) tend to show a remarkable decrease, especially in the radius.