Allergy to dental metal alloys has been reported to occasionally cause dermatitis, stomatitis, lichen planus and pustulosis palmaris et plantaris. According to Faraday's law of electrolysis, when electric current flows into an anode, cationic metal ions dissolve in parallel with the amount of the electric current. Therefore, when patients hypersensitive to metals have suffered from the above mentioned persistent dermatoses, measurement of voltages and electric currents around the dental alloys of the patient's oral cavities was deemed necessary, in order to investigate whether or not such dental metals have been supplied causative metal ions to the patients. For the investigation of electrochemical dissolution of metal ions; firstly, voltages and electric currents between the mucous membrane and standard dental alloy tips placed in the oral cavities of each 15 healthy volunteers and patients were measured. Secondly, the same study was performed with the mucous membrane and actually installed dental metals with 158 patients who showed positive reactions to dental metal series patch test allergens composed of 19 reagents. The results were as follows: 1) Voltages between the mucous membranes and standard metal plates placed in the oral cavities varied depending on the composition of the dental metal. 2) The above-mentioned voltages changed when various food were present in the oral cavity. 3) With the dental metals actually present in the metal-hypersensitive patient's oral cavities, the voltages between the adjacent mucous membrane and dental metals varied greatly, depending on the individual patient rather than on the types of metal. Certain alloys acted as cathodes with some patients, but as anodes with others. 4) Amalgam and silver alloys showed higher voltages and more electric current with the smaller ranges of variation than other kinds of alloys. Both of these alloys tended to act as anodes, therefore, the electrochemical dissolution of metal ions was expected to be higher than with other metals. 5) The voltages and electric currents were often higher between mucous membrane and metals than between metals. These findings indicate that the patient's own mucous membrane acts as cathode for the electrochemical dissolution rather than the dental metals. 6) Measuring the voltage and electric current in the patient's oral cavities is technically easy, and is considered to be valuable since the tendancy of electrochemical dissolution of metal ions from the dental metals is clearly demonstrated.