To investigate the biobeical effects of terbium (Tb), male mice were intravenously ad ministered with TbCl3 at 10, 25, or 50 mg Tb/kg. Time-course and dose-related changes in organ distributions of Tb were determined . More than 95 % of the Tb in blood was in plas ma, and the concentrations decreased rapidly. Contrary to normal pharmacokinetics, Tb con centrations in plasma were higher in the 10 mg/kg group than in the 50 mg/kg group. The concentrations after injection of 25 mg/kg were between 10 and 50 mg/kg injections. Tb was incorporated mainly in liver, lung, and spleen. In all groups more than 80% of Tb adminis tered were found in these three organs. Disappearance of Tb in these organs was very slow.Tb was also found in kidney, heart and other organs. Coincidentally, it was found that the Ca concentration was increased in organs in which Tb was incorporated. After administration of Tb (50 mg/kg) the Ca concentration, compared to the controls, was 70-fold in spleen, 20-fold in lung, and 6-fold in liver. There were highly positive correlations between Tb and Ca concentrations in organs. Excretion of Tb in urine was 0. 15 ～ 0. 3 % and that in feces was 1.7～12. 5 % for up to 7 days. These results indicate that liver, lung, and spleen are the main target organs of Tb administered intravenously, and that the increase in Ca concentrations is one of the important biological effects of Tb in target organs To investigate the biobectrical effects of terbium (Tb), male mice were intravenously administered with TbCl3 at 10, 25, or 50 mg Tb / kg. Time-course and dose-related changes in organ distributions of Tb were determined. % of the Tb in blood was in plas ma, and the concentrations decreased rapidly. Contrary to normal pharmacokinetics, Tb con centrations in plasma were higher in the 10 mg / kg group than in the the 50 mg / kg group. The concentrations after injection of Tb was incorporated mainly in liver, lung, and spleen. In all groups more than 80% of Tb administered were found in these three organs. Disappearance of Tb in these organs was very slow.Tb was also found in kidney, heart and other organs. Coincidentally, it was found that the Ca concentration was increased in organs in which Tb was incorporated. After administration of Tb (50 mg / kg) the Ca concentration, compared to the controls, was 70-fold in spleen, 20-fold in lun There were highly positive correlations between Tb and Ca concentrations in organs. Excretion of Tb in urine was 0.15-1.3% and that in feces was 1.7-12.5% ​​for up to 7 days. These results indicate that liver, lung, and spleen are the main target organs of Tb administered intravenously, and that the increase in Ca concentrations is one of the important biological effects of Tb in target organs