1. The kinetics and the autoradiographic distribution of ⁴⁵Ca and ¹³³Ba have been compared in ten-year-old beagle dogs killed seven days after injection.

2. The rate of long-term transfer from blood to the skeleton was measured at five days (A₅) and found to be two to three times larger for barium than for calcium.

3. It was found that this preference for barium could not be explained by differences in diffuse uptake.

4. Instead, the major difference between the nuclides appeared to be an increased uptake of barium on inactive bone surfaces.

5. Furthermore, the shape of the curve of ¹³³Ba specific activity in the plasma at very short times after injection was consistent with a passive diffusion from blood to the plane surfaces of bone and tissue. On the other hand, the shape of the ⁴⁵Ca plasma curve suggested that some kind of active transport may be counteracting the diffusion of calcium from blood to bone. Such active transport might be associated with the endosteal cells lining bone surfaces.

6. The ratio of the diffuse specific activity in the mid-shafts of the long bones to the average specific activity of the skeleton was only 1/6. However, a larger fraction of skeletal activity was probably associated with augmentational hotspots, increased uptake of ⁴⁵Ca in the relatively young bone which resulted from recent Haversian remodeling.

7. Measurements of the diffuse specific activity of ⁴⁵Ca throughout the skeleton showed variations of as much as a factor of 2.5 from one site to another. This variation correlated well with the total uptake of ⁴⁵Ca at that site and hence was closely related to metabolic activity. The implications of this finding to the role of the osteocyte and to theories of calcium homeostasis are discussed.