After inducing fluorine poisoning in two groups of young rats by administering various NaF solutions consecutively for one week in one group and for one month in another group, the auther observed the phosphorus metabolism in vitro of the teeth and bones of these rats by means of radioactive isotope P(32), and obtained the following results. On the other hand, the author investigated what change might be brought upon the phsophorus metabolism in the milk teeth of the children given a dose of 0.2 mg and 1.0 mg NaF each for the period of one year with the purpose to prevent tooth-decay, and compared the results of the two. Method: Both the rats with fluorine poisoning and control group are sacrificed and the teeth and bones are extracted. Also the milk teeth at the deciduous stage are extracted from kindergarten children treated with NaF for the prevention of tooth-decay. These teeth and bones are cleaned of tooth pulp and bone marrow as much as possible and pulverized. The pulverized materials are filtered through the strainer of 60 meshes and the materials from the teeth and bones are weighed exactly 20 mg each, and to each of these samples 2cc phosphoric acid (H(3)P(32)O(4), 3000 counts/min) is added in a test-tube and shaken for one hour. The sedimented substances are separed from the supernatant and thoroghly washed, filtered, and dried, and finally the radioactivity is measured by Gieger-Müller counters. Experimental results: 1. In the group of animals given NaF for one week the quantity of P(32) incorporated in the teeth tends to decrease slightly with the increase in the dosage of NaF, while the amount of P(32) incorporated in the bones rather tends to increase slighelv along with the increase in the dosage of NaF given. 2. In the group given NaF for one month the amounts of P(32) incorporated in the teeth and bones tend to decrease along with the increase in the dosage of NaF given. 3. Both in the group given NaF for one week and that for one month the amount of P(32) incorporated in the bones is greater than that in the teeth. 4. In the teeth of the kindergarten children rally administered with fluorine compound repeatedly for one year the amount of P(32) incorporated in enamel is clearly less than that in dentin along with the increase in the dosage of NaF.