We administered radon (Rn) to rabbits by inhalation and examined changes in the lipid peroxide (TBARS) level, superoxide dismutase (SOD) activity, and membrane fluidity in various organs to clarify the therapeutic effects of Rn. The lipid peroxide level of the brain was significantly decreased immediately after Rn inhalation for 90 min in both the low concentration group (about 7-10 kBq/liter) and the high concentration group (about 14-18 kBq/liter) as compared with that in the control group. It further decreased in the low concentration group but slightly recovered in the high concentration group 2 h after inhalation. The lipid peroxide level of the lung showed no change immediately after inhalation but decreased significantly in both groups 2 h after inhalation. With regard to SOD activity in the brain and lung, only that in the brain showed significant increase in the high concentration group immediately after inhalation; no other change was observed. Membrane fluidity, especially the fluidity of membrane protein, was significantly increased in the brains of both groups immediately after inhalation, and that 2 h after inhalation in the lung was significantly increased in both groups. These findings suggest that the inhalation of Rn at Rn springs contributes to the prevention of brain disorders related to peroxidation reactions by promoting these physiologic changes.