Three groups of rats were fed respectively on low-phosphorus diet (McCollum No.3143 rachitic diet, Steenbock No.2965 rechitic diet, and its modified form), high-phosphorus diet (Steenbock No.2965 rachitic diet containing Na_2HPO_4 instead of CaCO_3), and a control diet. Then, radioisotopic experiments were carried out on the metabolisms of phosphorus and calcium in teeth and surrounding tissues, and effects of vitamin D_2 administration on their metabolisms. Metabolisms of calcium and phosphorus in the dental hard tissues resembled each other in exchange and accumulation on the tissues as same as in the bone tissues. However, some essential differences in behaviors of these elements were observed. Furthermore, the ratio of calcium to phosphorus of the diet, parts of the teeth and the lapse of time were found to cause the difference between calcium and phosphorus metabolisms. Effect of vitamin D_2 administration on the dental hard tissue was greater in the high-phosphorus diet group than the low phosphorus diet one. This effect was not proportional to the amount of administered vitamin D_2 or frequency of administration. A prolonged administration of vitamin D_2 sometimes brought about a decline of metabolism of inorganic matters.