The effects of high frequency ultrasound on the inner ear structures after irradiation of the lateral semicircular canal have been analysed by means of light-and electronmicroscopy. Sudden as well as gradual morphological changes in the vestibular sensory epithelium are described. Damage to the connective tissue stroma, blood vessels and nerve fibres characterized the chronic phase. Both cochlear morphology and hearing remained normal after irradiation of some behaviorally conditioned animals. The temperature rise in the labyrinth during continuous irradiation was studied. The experiments indicated that the mechanical effects may be more significant than the thermal in explaining the structural changes of the inner ear of guinea pigs. The results are compared with previous extensive experience of ultrasound treatment of patients with Meniere's disease.