Damaged hair cells in the avian basilar papilla are replaced by regenerative proliferation of supporting cells and transdifferentiation of supporting cells into hair cells. In the mammalian vestibular system, transdifferentiation and, possibly, the repair of damaged hair cells appear to play significant roles. Several growth factors have been found to be associated with the regeneration/repair process: insulin, insulin-like growth factor 1 (IGF-1), and fibroblast growth factors are important for avian inner ear regeneration/repair, whereas epidermal growth factor, transforming growth factor alpha, insulin, IGF-1, and IGF-2 are important for regeneration/repair in the mammalian labyrinth. Increasing evidence suggests that regeneration/repair of mammalian auditory hair cells is possible during the early neonatal period and may exist to a very limited degree at later times.