Bisphosphonates (BP) have been clinically used for the treatment of bone resorption disorders, including postmenopausal osteoporosis, hypercalcemia of malignancy, and Paget`s disease of bone. Recently, ototoxicity was reported in association with BP treatment but its pathogenesis has not been elucidated. This study was carried out to evaluate ototoxic potentials of BP, especially when they pass into the inner ear. Each of three BP (pamidronate, alendronate, or risedronate) was surgically administered to the round windows of guinea pigs. The hearing thresholds were obtained by auditory brainstem response measurements just before and at 1 hour through 4 weeks after BP administration. The cochlear specimens were obtained from animals at the termination of experiment 4 weeks after administration and hair cell changes were examined with scanning electron microscope. In pamidronate group, hearing threshold was elevated to 13 dB nHL at 1 week, and then returned to a normal level at 4 weeks. In contrast, hearing threshold was elevated up to 35 dB nHL at 2 weeks in alendronate group, and 37 dB nHL at 2 weeks in risedronate group, respectively, and their hearing did not return to a normal level. Hair cell changes in organ of Corti were mild in pamidronate group, but moderate to severe in alendronate and risedronate group, respectively. The results of the present study suggest that the pamidronate would be a good candidate drug for local delivery to the ear.