Recently, more and more attention has been drawn to the injurious effects of radiation. Therefore, the effect of the internal use of radioactive waters was studied experimentally to decide if hazardous influence is practically provable through the spa treatment with natural radioactive waters. A radioactive water of Spring No. 49, Group A, Masutomi was used for the study as it holds the highest record of Rn content in Japan. The water used contained 720×10^-9c/l Rn-218×10^-9c/l Rn at the time of the experiment.<br>Fifty-seven rabbits in total were divided into 8 groups. Ten to thirty ml of the water was given daily to each rabbit by stomach tube for 150 days. Stored mineral water, tap water from Tokyo, or river water of Masutomi (Rn content: 1.3×10^-10c/l) were used as controls.<br>Blood picture, serum calcium, serum phosphor, and alkaline phosphatase activity were measured before the experiment was started, and repeated periodically after the 2, 4, 6 and 8 weeks and after 3, 4, and 5 months. After five months the animals were sacrificed and investigated histologically. α- and β- activities of the liver, spleen, lung, brain, heart, adrenals, testis, bone, and kidney were determined by using a gas flow counter and a Geiger-Müller counter.<br>22×10^-10c/l-238×10^-10c/l of Rn was present in the mineral water given at one time. Rn concentration of the water was equivalent to about 100-400 times “Maximum permissible concentration of Rn in waters for general use N. B. S.”<br>Results obtained were as follows:<br>1) Hemoglobin content and erythrocyte count showed, generally, a tendency to increase slightly or showed no change, and little difference was recognized among each group.<br>Platelets count was variable but a tendency towards an increase in two phases was seen especially in the group of rabbits drinking fresh mineral water at the Masutomi Spa.<br>The white cell count proved also variable but a tendency towards increase was verified both in the tap water group and in the mineral water group; the fresh mineral water group at the Masutomi spa, especially, showed the tendency towards the increase in two phases compared with the river water group. However, after three or more months, experiments revealed, that the white cell-count reached the initial level and no definite decrease in leucocytes was seen, even in the fresh mineral water drinking group.<br>Though lymphocyte and pseudo-eosinophil counts also showed a tendency to increase, no significant difference was generally proven between the mineral water group and the control groups.<br>2) No significant difference was obtained between the changes in the control groups and mineral water groups concerning serum calcium, serum phosphorus (inorganic phosphorus, acid soluble phosphorus, total phosphorus), and serum alkaline phosphatase activity.<br>3) Through measurement of radioactivity of the ashes of the organs, a weak α-activity was discovered especially in the bone, which was estimated to be far less than the maximum permissable concentration of Ra.<br>4) Histological investigation of the liver, kidney, spleen, heart, testis, adrenal, brain and bone marrow revealed no characteristic findings of injuries that might result from the radioactivity.<br>Therefore, for the present, no positive reason was obtained against spa treatments using natural radioactive springs in Japan.