The intrathecal administration of gentamicin has been used to treat bacterial meningitis. However, the influence of this route of administration on the well-known ototoxic activity of gentamicin is unknown. This route of administration should facilitate access to the inner ear and should increase the ototoxic liability. The possible ototoxic and neurotoxic effects of intrathecally administered gentamicin (0.5, 1, 2, 4, 6, 10 mg/day) in cats were evaluated. Hearing thresholds were obtained by a noninvasive technique prior to and on the 7th and 21st days after the initiation of a seven-day dosage regimen. Histological examination of the organ of Corti was routinely done. Additionally, the vestibular organs of one animal on the 4 mg/day regimen were examined. The concentrations of gentamicin in the cerebrospinal fluid were also determined. The data indicate that the 2 mg/day dose may be very close to the threshold dose for ototoxicity when gentamicin is given by intracisternal administration.