In order to study the effect of the thiamine derivative O, S-dicarbethoxythiamine (DCET) on kanamycin (KM) ototoxicity, electrophysiological and phase contrast microscopical studies were carried out using rabbits with electrodes permanently implanted on the round windows. The main electrophysiological findings were as follows: after the onset of reduction of cochlear microphonic amplitude, in the group receiving l00 mg/kg of KM and in the group receiving 100 mg/kg and 20 mg/kg of thiamine chloride, the curves of cochlear microphonic amplitude were reduced rapidly. On the contrary, in the group receiving 100 mg/kg of KM and 20 mg/kg of DCET, the curves were reduced with a gentle slope to a lower level. There was a clear-cut difference between the curves of the two former groups and those of the latter group. The histopathological proof on the effectiveness of DCET as regards KM ototoxicity could not be obtained by phase contrast microscopy. Based on electrophysiological differences, it seems to be quite conclusive that DCET is able to prevent progression of the functional damage caused by KM ototoxicity, and that some experimental support to clinical applications regarding KM ototoxicity has been given.