Chlorhexidine  digluconate is a derivative of chlorhexidine and is a popular disinfectant with a strong bactericidal action that is widely used for preoperative sterilization in ear surgery. The purpose of this investigation is to ascertain the potential ototoxicity of this agent. After topically applying chlorhexidine  digluconate solutions to the middle ear cavities of 12 cats, we observed the excised cochleas using both scanning and transmission electron microscope studies. Either 0.05% or 2% chlorhexidine  digluconate solutions were infused into the right ear of the test animal through one of two tubes chronically installed in the tympanic bullae. The left ears were utilized as controls and were infused with sterilized physiological saline. The solutions were administered once every other day for three separate infusions. Nine animals were decapitated 7 days after the third application, while the other three animals were sacrificed at 4 weeks. In the 2% chlorhexidine  digluconate group, we found that hair cells in the organ of Corti had degenerated and had lost their hair bundles over a wide range. This pathology was more marked in the lower cochlear turns. In the animals sacrificed at 4 weeks, the injuries present seemed to have progressed. Even at a clinical concentration of 0.05%, chlorhexidine  digluconate caused intracellular degeneration but with little surface damage. Our findings would suggest a cause of hearing loss when chlorhexidine  digluconate is used clinically in the ear.