Cell proliferation in the cochleae of guinea pigs and rats was investigated after systemic application of kanamycin sulfate (KM) and ethacrynic acid (EA). Bromodeoxyuridine (BrdU) was injected daily for 10 days, after which the number of BrdU-positive cells was counted in paraffin sections of the cochlea. Only a few BrdU-positive cells were present in the spiral ligament and among the acoustic nerve fibers in the non-deafened control animals. Animals treated with KM and EA had profound hearing loss and significant increases in the number of BrdU-positive cells in the spiral ligament and among the acoustic nerve fibers. No BrdU-positive cells were found in the auditory sensory epithelium of any animal. These findings suggest that in the mature mammalian cochlea cell proliferation increases in nonsensory regions after ototoxic damage but may not occur in the auditory sensory epithelium.