By oral administration of various NaF solutions (drinking) to young rats and feeding them under the same conditions for 50 days, the author measured the tail length and body weight of these animals in different groups in order to see the influences of fluorine on their growth and investigated them experimentally; and obtained the following results. Along with this experiments the author also gave orally pills of cod-liver oil containg 0.2 mg NaF to kindergarten children repeatedly for one year with the purpose to prevent their tooth-decay and examined their height, body weight, and the percentage of incidence of tooth-decay. Mesthod: 1. For the experimental animals young rats of about forty days old and weighing about 50 g were selected, and they were divided into four groups each consisting of five males and five females to the total of ten in each group. They were made to drink freely NaF apueous solution of various concentrations at 1000 ppm, 500 ppm, 100 ppm, 10 ppm, and 2 ppm. The duration of the drinking of NaF was for 50 days. Namely, observations were limited to the stage of the most active growth and changes were observed in that period. 2. As for the drinking expeariment of fluorine with kindergarten children, sugarcoated cod-liver oil pillets containing 0.2 mg NaF, 0.02 g cod-liver oil, and 500 γ vitamin B(1) were used. These pillets were given to the children on every school day for one year under a controlled supervision with exception of sundays, holidays, summer and winter vacation. Their height and body weight were measured once every month, and the rate of incidence of tooth-decay in these children was examined twice, namely, once before the start of NaF-administration and once immediately after the cessation of the administration. Results: 1. Although there can be recognized no great difference in the change of the tail length in the growing young rats given NaF aqueous solution by drinking at a low concentration as compared with the control, there is a slight accelerating tendency. In the group of rats given large amount of NaF solution by drinking in comparison with the control there can be recognized no marked difference up to 30 day after the start of taking fluorine compound, but thereafter the rate of tail growth tends to be retarded and by the fiftieth day the tail growth is diminished as compared with the control. 2. In the groups of young growing rats given orally a low concentration of NaF solution their body weight increases normally. However, in the groups given orally a large amount of NaF solution, although no great change can be recognized up to the 30 th day after the start of the experiment, thereafter a decrease in the body weight can be recognized. 3. In the case of the kindergarten children the oral administration of fluorine in the minimal amount shows a favorable effect on the increase in their height and body weight as compared with the control group without such a treatment. 4. Judging from the rate of the incidence of tooth-decay in the kindergarten chileren as compared with that in the children of the control school where no such treatment was undertaken. the incidence of tooth-decay in these children is far lower than in the control school children. This fact indicates that the oral administration of fluorine compound is effective in preventing the tooth-decay in young children of the kindergarten age.