Damage caused by neomycin sulphate solution, identical in concentration to that in ear drops, was tested on the cochleas of 28 guinea pigs. Neomycin sulphate with dexamethasone sodium, and physiologic salt solutions were also tested. Only neomycin solutions damaged the cochlea, and even after only a single exposure. The damage after a single exposure was to the supporting cells and not to the hair cells. However, because of the importance of the supporting cells for normal hearing and because destruction of supporting cells leads to shedding of hair cells, the resulting functional impairment is similar. As a result of further exposure to neomycin the hair cells themselves are damaged, and there is also loss of cell hair. Damage to either the cells or to the hair causes hearing loss. From our work it is clear that in guinea pigs neomycin is absorbed from the middle ear into the inner ear and damages the cochlea. Increased exposure to neomycin increases the damage.