The inner hair cells and their nerve endings in the basal and third turns of the guinea pig cochlea were reconstructed three-dimensionally from serial thin sections by means of computer graphics. The inner hair cells showed narrowing below the cuticular plate and gradual increase in size toward the base where the surface was indented by nerve attachments. The sizes of the afferent nerve endings were in the range of 0.9 to 3.2 microns in cross section. Large endings were fewer and were located mainly on the pillar cell side, whereas small endings were numerous and were located on the modiolar side. The number of afferent nerve endings on a single cell in the basal turn averaged 26 and in the third turn, 17. All the afferent fibers were unbranched, most of them innervated a single cell, and 81% of them formed synapses with distinct synaptic bodies in the inner hair cells. A few afferent endings formed membrane specializations with two adjacent inner hair cells but the synaptic body was seen in only one cell. The number of efferent endings, from 0.08 to 2.0 micron in diameter, was about nine in the basal turn and was seven in the third turn. No significant difference was seen between the basal and third turns in cell shape, proportion of afferent to efferent endings, nor in morphology of synapses.