The author studied the changes of catecholamine levels of the whole brain of male mice weighing 15-20g before and after a single bathing for 20 minutes, 37±1℃ in temperature, in the radioactive hot spring in Misasa (Rn content : 74.83×10(-10)C/l), and compared the results obtained with those of the plain hot water bathing. The catecholamines (dopamine and noradrenaline) were measured by gaschromatographic method using electrone capture detector. It may be summarized as follows : 1. No significant change of dopamine and noradrenaline levels of the whole brain of mice was recognized before and after the plain hot water bathing used as a control. 2. Noradrenaline levels of the whole brain of mice had increased following the radioactive hot spring bath, and remained at same levels for 30 minutes after bathing. Dopamine levels had no change immediately after bathing, but had lowered at 30 minutes from the time of the bath. 3. Causes of these results are under investigation.