Left unilateral cochlear lesions were performed on 26 albino rats at 1.5 months of age. After survival times ranging from 1 h to 6 months, the animals were perfused via the aorta with mixed aldehydes. Blocks including the cochlear nerves were removed, embedded in Araldite, sectioned in a plane transverse to the longitudinal axis of the nerve, and analyzed in the light microscope. Degenerating fiber profiles were grouped into 4 categories, and their relative frequencies were counted, as were numbers of normal fibers and glial cell nuclei. The cross-sectional areas of the nerves were measured. Lesion extent was evaluated by means of sections through operated cochleas from short and long survival times, and right cochlear nerves from 11 of the animals were used as controls. In the left nerves, segmental swelling of fibers occurred as early as 16 h survival, followed by collapse of fibers and breakdown of myelin sheaths. Starting at 36 h survival, increased numbers of glial cells were seen in the nerve. At longer survival times there were decreases in the cross-sectional area of the nerve and in the packing density of degenerating fiber profiles. At the longest survival times, a substantial amount of debris remained which resembled that seen in early stages. Finally, there was evidence of continued loss of nerve fibers occurring over a period of weeks to months.