Exposure to water in hot springs containing thoron is thought to exercise beneficial effects on hypertension and diabetes mellitus. To put to a test this hypothesis we examined the time dependent changes in the levels of lipid peroxide, vasoactive- and diabetes associated substances in human blood in order to throw further light on the possible beneficial influence of thoron and thermal therapy on the mechanism of hypertension and diabetes mellitus. Every 2 days, nasal inhalation of vapor containing thoron was performed for 40 min. Blood samples were collected after each treatment at 1, 2, and 3 weeks after the first treatment. Results show that the treatment decreased the lipid peroxide levels. The finding suggests that the treatment contributes to the prevention of peroxidation reaction related to hypertension and diabetes mellitus. Moreover, the changes in vasoactive-associated substances indicate an increase in tissue perfusion, suggesting that the treatment plays a role in alleviating hypertension. The treatment decreased the total ketone body levels and the finding suggests that the treatment contributes to the prevention of diabetes mellitus related to the insulin deficiency.