The effects of streptomycin sulfate (SM) and kanamycin (KM) on the sensory epithelia of the inner ear in guinea pigs were studied functionally by the rotatory test and the pinna reflex test, and surface specimens of the vestibular organ and organ of Corti of the animals were also studied. For the histochemical study, perfusion of the vestibular and spiral organs with reaction, solution for succinate dehydrogenase staining was carried out in the temporal bones removed immediately after decaptation. And their enzymic activities of the hair cells were investigated in surface specimens from the cristae arnpullares, macula utriculi and sacculi and organ of Corti. The present study revealed that SM caused more remarkable damage on the vestibular hair cells, especially type I of hair cells, than on the hair cells of organ of Corti. On the other hand, KM induced severer damage on the hair cells of organ of Corti, especially outer hair cells, than on the hair cells of the vestibular organ. In those animals with extensive and severe decrease in enzymic activity of the hair cells, there was a relatively close correlation between the result of rotatory test and the hair cell damage of the vestibular organs, and also between the frequency range of hearing loss estimated by the pinna reflex test and the extent of outer hair cell damage. However, in these animals with less extensive and mild decrease in enzymic activity of the hair cells, the author couldn't found the remarkable correlation between the results of the functional tests and the morphological findings. In conclusion, the author emphasized that the rotatory test as well as the pinna reflex test combined with histochemical investigation of enzymic activity of the hair cells of the vestibular organ and organ of Corti were very valuable for the evaluation of ototoxic effects of antibiotics on the vestibular organ and organ of Corti.