Prolonged studies of the rate of cortical bone formation in three adult dogs using double tetracycline labeling (two twelve-week periods of labeling twelve weeks apart) are reported. These studies, covering twenty-four weeks in a thirty-six weeks period, provided the first long-duration, direct measurements of rates of cortical bone formation. The double labeling also permitted evaluation of the variations in the rate of bone formation with respect to time and space.

Spatial and temporal variations in skeletal metabolic activity are marked. Neither one section nor four sections from one skeletal site may be considered representative of the entire skeleton, even when cortical bone formation in a given site or section is measured over 168 days. The activity in homotypic sections and homotypic sites also varies and any comparisons of such sections and sites must be interpreted with caution.

The data presented are compatible with the hypothesis that cortical new-bone formation in one long bone of an adult animal, measured over six months, is representative of cortical-bone growth in long bones in that animal. For labeling periods of twelve weeks it is necessary to measure sixteen sections from each of five long bones from one side of the dog to obtain representative data for new-bone formation in long bones.

Studies of porosity showed highly significant differences between dogs, between sites, and between different long bones, but no significant differences between sides in the same dog or between ribs.

Ribs differ significantly from long bones in metabolic activity not only in the rate of new-bone formation, but also in the amount of variability in rate of formation and in porosity.

The rates of bone formation derived from this study indicate that the over-all annual rate of cortical bone formation in the long bones varied from 5 to 11 per cent in three mature adult dogs, and that the over-all average cortical bone formation rate for ribs in these same three dogs varied from 14 to 44 per cent.