The author investigated the changes in skin temperature, oral temperature, blood pressure, pulse rate, respiration rate and weight of body before and after a radio-active thermal bath in Misaswa, and compared the results with the corresponding changes after a plain water bath at a same temperature (43℃) with same duration (5min.). A rise in skin temperature after the thermal bath durated longer than after the control plain water bath. Changes in oral temperature did not correspond to the changes in skin temperature and to the suhjective warm feeling. Changes in the pulse pressure showed no significant difference between the radio-active thermal bath and plain water hath. But a more prolonged lowering of the minimal blood pressure was proved after a radio-active thermal bath than after a plain water bath at same temperature with same duration. The response in blood pressure to thermal bath was more marked in winter than in summer. A rise in pulse rate was recognized after all kinds of hot baths, but no difference was proved between them. A slight acceleration of respiration rate and a loss in weight were observed after hot bath at 43℃ for 5 minutes. But these effects were slighter after radio-active thermal bath than after plain water bath at same temperature and same duration.