After the administration of Ca45 and P32 to an adult dog the rates of uptake of calcium and phosphorus were studied in morphologically well defined microscopic bone structures, including single Haversian systems and selected portions of lamellar bone carefully dissected free from adhering tissues, such as the surrounding bone and blood constituents in the Haversian canals. The structures were dissected at various time intervals during an experimental period of fifteen days. The uptake of calcium and phosphorus expressed in per cent per hour occurred in the same proportion as the concentrations of these elements in the bone structures. in the Haversian systems the rates of uptake were related to the degree of mineralization. This correlation between rate of uptake and degree of mineralization was not found, however, on comparing different types of bone structures.