1. Atrophy of bones of young rats was produced by feeding a diet low in calcium content.

2. The healing of fractures of atrophic bones produced by a diet low in calcium progressed to the stage of complete bony union at a rate comparable to that observed in animals of a control series fed a normal stock diet.

3. The total amount of bone laid down in repair was less and the new trabeculae and cortices of the bone were definitely thinner in those animals fed the deficient diet than in those constantly receiving a diet adequate in calcium.

4. Addition of an excess of vitamin D or calcium carbonate, or both of these elements, to the diet of rats, whether they had been previously on a diet low in calcium or on a normal stock diet, did not hasten the rate or improve the quality of healing of the fracture, as compared with the fracture healing of rats constantly fed the normal stock diet.

5. Some evidence is presented to indicate that massive doses of calcium or of calcium and vitamin D by mouth may produce an inhibiting effect on the quality of healing of fractures of normal or atrophic bones.

6. Since the vitamin-D requirement for the average human adult is not known, and since massive doses of vitamin D may produce atrophic changes in the bones of experimental animals and delayed healing of fractures, we question the advisability of continuing the practice now becoming common of prescribing massive doses of vitamin D, or of vitamin D and calcium, for patients with fractures, in whom there is no evidence of a deficiency of these elements.

7. There is a real need of a carefully planned and executed longtime study to determine the vitamin-D and calcium requirements of the average adult human.