Young Wistar female rats weaned at 25 days were placed on a control diet, or a diet extremely deficient in phosphorus but adequate in all other respects. After five weeks on the diet they were injected with 10 microcuries of high specific activity radiocalcium. The animals were killed at varying periods after injection and samples of bone, teeth and soft tissues were taken for chemical, radio-isotope, and histological analysis. The phosphorus deficient animals showed a marked demineralization of the skeleton, lower radiocalcium uptake by bone and higher radiocalcium uptake by the teeth. The accretion and resorption rate of bone in the phosphorus deficient animals was markedly reduced. The resorption rate, however, was higher than the accretion rate and accounted for the reduced mineralization of the bone. The femur of the rachitic animal had an exchangeable calcium portion of 13% as compared to 4-8% in the control animal. The teeth of the phosphorus deficient animal showed a reduced accretion and attrition rate, and a statistically evident difference in the chemical calcium and phosphorus content. The accretion rate was higher than the attrition rate, so that the teeth remained well mineralized. The depression of the accretion rate was not as marked as that observed in the bone, therefore, the marked demineralization of the rachitic animals' bones was not evident in the teeth. The serum plasma levels for calcium and phosphorus were 9.43 mg.% and 2.86 mg.% in the rachitic animal and 9.91 mg.% and 7.24 mg.% in the control animal. The disappearance of plasma radiocalcium was not as rapid in the rachitic animals. Starvation of phosphorus deficient animals resulted in a lower plasma calcium and raised plasma phosphorus level similar to that observed in parathyroidectomized animals. The soft tissue calcium concentration in the rachitic animals as compared to the control animals was higher for all soft tissues examined with exception of the kidney and blood plasma where there was no significant difference. The amount of calcium in the various muscle compartments was calculated. The rachitic animal had a higher intra-cellular calcium concentration and the same extra-cellular concentration when compared with the control animal. There was no difference in the phosphorus concentration of the control and rachitic animals' soft tissues. Histological studies of the femur of the rachitic and control animal showed that the rachitic femur had a wider epiphyseal cartilage which was not uniform in width. The bone trabeculae showed wide irregular seams of uncalcified osteoid matrix. Histological and calcium analysis of the kidneys of the phosphorus deficient animals showed no evidence of calcium deposits or nephrocalcinosis. Histological studies of the parathyroid glands of rachitic animals showed a decrease, in the volume of the glands, in the size of the nuclear surface, and in the amount of cytoplasm present, when compared to the glands of the control animals. This study of calcium kinetics in the phosphorus deficient animal coupled with the histological findings shows the possibility that phosphorus deficiency in rats produces a hypoparathyroid condition as a homeostatic mechanism to conserve phosphorus for the soft tissues. / Medicine, Faculty of / Cellular and Physiological Sciences, Department of / Graduate