The membrane potential difference with the living human skin between the physiological saline and the mineral waters was measured. The hypotonic mineral waters, that have a pH higher than the isoelectric point of human epidermis, showed a positive potential against the physiological saline. The membrane potential difference of these mineral waters against the physiological saline at the same temperature tends to increase by raising the water temperature. The acid waters showed either a positive or a negative potential against the physiological saline. The membrane potential difference between the mineral wates at room temperature and at the temperature raised above it is increased according to their temperature difference,