Ten guinea-pigs were treated with kanamycin 400 mg/kg for a 10-day period. Electrophysiological investigations were carried out, and the animals were killed one month after the end of the treatment for electron microscope study of the second turn of the cochlear spiral. The microphonic potential was considerably reduced, and the stria vascularis was the seat of lysosomal inclusions in the marginal and intermediate cells associated with myeloid bodies in the vascular endothelium. There was a gradient of lysosomal catabolic residues, probably lipoproteins, from the vascular sector to the cochlear canal; some of these products passed into the cochlear canal. Alterations of the hair cells in the second spire appeared to be minimal. It is hypothesized that damage to the stria vascularis precedes damage to the hair cells.