1.Eight groups of weanling rats were maintained for 60 days on diets containing calcium and phosphorus in the ratio 0·1 at four different levels of mineral, namely 0·08, 0·12, 0·23 and 0·32% Ca, and in the presence and absence of added ergocalciferol.
2. Provision of vitamin D increased final body-weight, whereas at the highest mineral intake body-weight was reduced.
3. At each level of dietary mineral the serum Ca concentration was raised by vitamin D, whilst the serum P concentration was lowered except when the mineral intake was lowest.
4. With increasing mineral intake the serum Ca increased until at the highest intake it declined. Serum P was increased only at the highest mineral intake.
5. Measurements were made of bone weights, ash content and volume. Provision of vitamin D increased the fresh weight of bone at all levels of dietary mineral, and the dry weight and ash content were increased except at the lowest level of mineral intake. In general, the higher the dietary mineral the greater was the bone weight and its ash content.
6. The presence of vitamin D resulted in an increase in tooth mass and ash content at each level of mineral intake. Tooth mass and ash increased with increasing dietary mineral except at the highest level when there was a decline.
7. At the lower levels of mineral intake the ratio of tooth mass to bone mass was greater than at the higher levels of intake.
8. Histological examination of the bones revealed no increase in the width of the epiphyseal cartilage in any group. In the group with the lowest levels of Ca and P without vitamin D (0·08% Ca, 0·8% P) the shafts of the femurs appeared thinner and porotic, and the trabeculae were thin; in the group fed the same diets supplemented with vitamin D the shafts were even thinner but less porotic; in all other groups the histological appearance of the bone was within normal limits.
9. Changes in the appearance of the dentine were demonstrated in all groups.