Thirteen chinchillas were exposed for 24 h to a 0.5-kHz OBN at 95 dB SPL. The cochleae of 8 animals were fixed at 0-d post-exposure; 5 were fixed after 1-2 wks of recovery. To keep cellular debris from washing away, all cochleae were plastic-embedded before being dissected into flat preparations. By phase-contrast microscopy, hair-cell losses were determined from apex to base. Damage consisted of scattered loss of OHCs in the apical half of the organ of Corti (OC) & small focal lesions (i.e., > 50% hair-cell loss over at least 0.03 mm) in the basal half. These specific patterns of loss suggest that noise damaged the apex & base by different mechanisms. In order to estimate the timing of cell loss, differential counts of missing & severely injured cells were performed. The presence of immature phalangeal scars & necrotic, oncotic & apoptotic hair cells indicates a recent loss while mature phalangeal scars indicate a long-standing loss. In the apical half of the OC in both groups of animals, many of the phalangeal scars replacing the missing OHCs were immature. Cellular debris was seen in the OC fluid spaces beneath these scars. By TEM, the debris was found dispersed in the Nuel spaces & consisted of vesicles of various sizes, small granules, shrunken & swollen mitochondria &, rarely, fragments of plasma membrane. TEM also revealed the presence of cellular debris in the endolymphatic space near the reticular lamina. This latter finding indicates that the barrier function of the reticular lamina broke down temporarily when the hair cells degenerated, before phalangeal scars formed. Debris in the Nuel spaces was not surrounded by plasma membrane as would be the case if the hair cells had been apoptotic before they died & then formed apoptotic bodies. In the basal half of the OC, focal lesions were found in 2 of eight 0-day & 3 of five 1-2 wk animals. In the 0-day ears, oncotic OHCs were found at the edges of the basal-turn lesions. In the 1-2 wk ears, the basal-turn lesions primarily consisted of immature phalangeal scars. The appearance of debris in the apex & base suggests that many of the OHCs were oncotic rather than apoptotic before they disappeared.