Rabbits were used as experimental animals. Total calcium contents in serum and weights of rabbits were determined. When 1 mg/kg, 5 mg/kg, 10 mg/kg, 20 mg/kg, 30 mg/kg and 40 mg/kg of NaF were administered for 10 weeks. (1) Total Calcium contents in serum (a) When small amounts of fluorine (1 mg/kg and 10 mg/kg of NaF) was administred, total calcium contents in serum increased. (b) When 20 mg/kg, 30 mg/kg and 40 mg/kg of NaF were administered, amounts of calcium in serum decreased remakably. (2) Weights of rabbits It was found that rabbits administered with 1 mg/kg, 5 mg/kgand 10 mg/kg of NaF tend to gain weight and to loss when 20 mg/kg, 30 mg/kg and 40 mg/kg of NaF was given.