Chronic effects of noise or kanamycin on the acoustic intra-aural reflex in nonanesthetized rabbits were studied. The intra-aural reflex activity was simultaneously recorded in both ears upon alternate stimulation of the left and the right side. The inner ears were analyzed in scanning and transmission electronmicroscopy. Exposure to bandlimited high-level noise was found to induce extensive deformations in the sensory hairs of the inner hair cells without loss of outer hair cells. Kanamycin caused a degeneration of outer hair cells in the basal 1-2 turns without affecting the morphology of the inner hair cells. The morphological changes, as a function of the location on the basilar membrane, were compared to physiological changes, as a function of frequency. In the noise-exposed animals the threshold of the intra-aural reflex was found to be raised in a frequency range corresponding to the extent of abnormal sensory hairs of the inner hair cells. In the kanamycin-treated animals reflex changes correlated to the loss of outer hair cells. The observations were interpreted as indicating that both types of hair cells cooperate in the activation of the intra-aural reflex in rabbits.