Various stages of changes in the nerve fibers, spiral ganglion cells, and satellite cells from the guinea pig cochlea 3 to 137 days after perilymphatic perfusion with streptomycin solution (2 and 20%) were observed electron microscopically. Initially, the axoplasms of the cochlear nerve fibers became swollen or pyknotic. Then, the axons disappeared and myelin lamellae disrupted. The Schwann cells shrank and degenerated, though their basement membranes survived for a time. Regeneration of the cochlear nerve fibers began with extension of axonal sprouts into the tube of the basement membrane and surviving Schwann cells, which still contained myelin debris. Only one of the axonal sprouts matured for myelination. These regenerating cochlear nerve fibers were found in the osseous spiral lamina, modiolus and internal auditory meatus, but these fibers atrophied and disappeared afterward. Retrograde degeneration occurred in the olivo-cochlear bundle. Some of the efferent myelinated fibers also showed temporary regeneration.