Much work on the relation of the disappearance of serum bone-seeking elements and skeletal uptake of these elements was conducted prior to 1955-for example, Ca45 in rats by Norris and Kisieleski (1), Ca45 and Na22 in dog by Armstrong et al. (2), and work on rabbits by Thomas et al. (3). In 1955, however, Bauer et al. (4) were able to describe and relate the disappearance of serum Ca45 to its uptake in bone by means of equations which allowed for the calculation of the accretion rate and exchangeable pool. Since that time, accretion and exchange of various bone seekers have been reported by various workers on a variety of animals under many experimental and clinical conditions (5). In many cases, Sr5 has been used instead of Ca45 in the study of bone mineralization (6-9). In spite of the numerous studies of accretion rate and exchangeable fraction, little attention has been devoted to distinguishing accretion and exchange of minerals in the trabecular and cortical bones per se. The present report deals with the determination of the accretion rate (A) and exchangeable fraction (E) in normal trabecular and cortical bones, in callus, and in devitalized implanted homograft. In addition, several sections of the hind limb of dog as well as the whole dog were studied, and their overall accretion rates and exchangeable fractions were interpreted on the basis of the individual A and E values of the several types of ash which make up these sections. This report is an outgrowth of a study which will be reported separately. The purpose of that study was to determine if devitalized, implanted homograft of bone can become a site for new bone formation.