WE investigated the potential for hair cell regeneration in neonatal rat organs of Corti grown in culture following destruction of hair cells by neomycin toxicity. Replacement hair cells were observed by light and scanning electron microscopy in lesion sites in the cultures treated with transforming growth factor-α, epidermal growth factor or a combination of both factors for 5–7 days post-injury. These new cells had morphological characteristics of immature hair cells. Autoradiographic localization of [3H]thymidine-labelled cells on semi-thin sections indicated that these replacement hair cells did not arise through renewed mitotic division, although an important mitotic proliferation was observed outside the area of supporting and sensory cells.