Hair cell regeneration is well documented in the inner ear sensory epithelia of lower vertebrates and birds and may occur in the vestibular organs of mammals. By contrast, hair cell loss in the mature mammalian cochlea is considered irreversible. However, recent reports have suggested that an attempt at hair cell regeneration could occur in vivo in aminoglycoside-lesioned cochleas from neonatal rats. After amikacin treatment, atypical cells with apical specialization reminiscent of early differentiating stereocilia are transiently present at the apex of the intoxicated cochleas but fail to differentiate as hair cells in later stages. In the present study, we used electronic microscopy, histochemistry, and confocal microscopy to investigate the cellular rearrangements in the amikacin-lesioned organ of Corti of rat pups. In addition, we used 5-bromo-2'-deoxyuridine immunocytochemistry to determine whether mitotic processes are involved in the formation of the atypical cells. The morphologic and molecular data suggest that atypical cells are not recovering hair cells, but share characteristics of immature hair cells and supporting cells. Proliferative cells were absent from the region occupied by atypical cells, suggesting that the latter did not arise through mitotic processes. Altogether, the present results support the hypothesis that atypical cells arise through direct transformation of some of the supporting cells that reorganize during hair cell degeneration.