1. Eight groups of ten weanling rats were maintained for 60 days on diets containing calcium and phosphorus in the ratio 10:1 at four different levels of mineral, namely 0.8, 1.19, 2.29 and 3.33% Ca.
2. At the two lower levels of mineral intake the provision of vitamin D reduced final body-weight. Increasing the mineral intake increased final body-weight except at the highest level.
3. Increasing the dietary mineral content had no effect on serum Ca, but serum P was higher in the groups receiving the two higher levels of dietary mineral. Vitamin D raised the serum Ca level in the rats receiving the two lower levels of mineral, and serum P was raised by the vitamin at all levels of mineral intake.
4. Provision of vitamin D at the two lower levels of mineral intake decreased the fresh weight, dry weight and volume of the humerus but had no significant effect on the absolute amount of mineral ash in the bone. Consequentially there was an increase in percentage of ash, overall density and the ratio of weight of ash to organic matter in dry fat-free bone (A:R value). With increasing mineral intake there was an increase in all the bone measurements except volume.
5. The presence of vitamin D had no effect on tooth mass or ash content. Increasing the mineral intake caused an increase in tooth mass and ash except at the highest level of intake.
6. The ratio of tooth ash to bone ash was very much greater at the lower levels of mineral intake than at the higher levels.
7. In the animals on the two lower levels of mineral intake the bones were frankly rachitic, and the presence of vitamin D ameliorated the condition. With increase in mineral intake histological signs of rickets were reduced, but even at the highest level of dietary mineral the epiphyseal discs appeared slightly wider than normal.
8. The incisor teeth of animals on the two lower levels of dietary mineral had wide predentine, and the presence of vitamin D reduced the amount of predentine. At the two higher levels of mineral intake the dentine appeared normal when vitamin D was present in the diet.
9. Quantitatively, the bones were much more affected than the teeth by a low intake of mineral at a Ca to P ratio of 10:1.