The microhardness-testing technique was applied to the study of the mineralization rate in the spongiosa of shaft bones of four dogs of various age. When bone tissue of the same “age” is tested the results indicate a higher mineralization degree of the metaphyseal than of the epiphyseal trabeculae; the degree of the mineralization of the latter is nearly equal to that of the osteons of the diaphyseal and metaphyseal cortex. The mineralization rate appears to decrease with the age of the animal both in spongy and in compact bone.