The cochlear toxicities of six aminoglycoside antibiotics were functionally and morphologically evaluated in guinea pigs. Forty-six Hartley strain guinea pigs were allocated into seven groups, including a control, and given intramuscular injections of one of the drugs or saline solution daily for 28 days. The hearing thresholds of all the animals were measured by the differential frequency pinna reflex test before, during, and after treatment to assess changes in cochlear function. Scanning electron microscopy was used to examine morphological changes in the cochlea after treatment and to detect degenerative changes in the auditory hairs of the basal turn. The results obtained from the functional and morphological examinations were well correlated, indicating the most severe changes in animals given gentamicin followed, in order, by amikacin, tobramycin, kanamycin, dibekacin, and ribostamycin.