Morphological changes of the eighth nerve were observed in the guinea pig between 1 month and 1 year after treatment with large doses of the antibiotic amikacin which resulted in complete cochlear hair cell destruction. The neural retrograde degeneration was found to be relatively fast, with a considerable loss (30 to 55%) of ganglion cells one month after treatment, continuously increasing (up to 85) after one year. Gross changes in the habenula perforata and in the spiral ganglion are described, together with ultrastructural alterations of organelles important for the cell metabolism and axonal transport. The rapid degeneration and the morphological findings suggest a direct influence of toxic substances on the ganglion cells.