The technique and some preliminary results of the application of high-voltage electron microscopy (HVEM) to the study of inner ear morphology in the guinea pig are reported in this paper. The main advantage of HVEM is that sharp images of thicker specimens can be obtained because of the greater penetrating power of high energy electrons. The optimum thickness of the sections examined with an accelerating voltage of 1,000 kV was found to be between 500 to 800 nm. The sections below 500 nm in thickness often had insufficient contrast, while those above 800 nm were rather difficult to interpret due to overlap of images of the organelles. The whole structure of the sensory hairs from the tip to the rootlet was more frequently observed in the 800-nm thick sections. Thus the fine details of the hair attachment to the tectorial membrane as well as the hair rootlet extension into the cuticular plate could be thoroughly studied in the HVEM. In specimens fixed in aldehyde containing 2% tannic acid, the attachment of the tips of the outer hair cell stereocilia to the tectorial membrane was observed. For the inner hair cells, however, the tips of the hairs were separated from the undersurface of the tectorial membrane. The majority of the rootlets of the outer hair cells terminated at the midportion of the cuticular plate, while most of the inner hair cell rootlets traversed the entire width of the cuticular plate and extended into the apical cytoplasm. These differences in ultrastructural appearance may indicate that the two kinds of hair cells play different roles in the acoustic transduction process. The three-dimensional arrangement of the nerve endings on the hair cells was also studied by the serial thick-sectioning technique in the HVEM. In general, an entire arrangement of the nerve endings was almost completely cut in less than ten 800-nm thick sections instead of the 50- to 100-ultrathin (ie, less than 100 nm) conventional sections for transmission electron microscopy. The present study confirms an earlier report that the first row outer hair cells in the third cochlear turn are innervated by nearly equal numbers of efferent and afferent endings, the average number being nine.