Hair cells, the sensory receptors of the mammalian inner ear, have long been thought to be produced only during embryogenesis, and postnatal hair cell loss is considered to be irreversible and is associated with permanent hearing and balance deficits. Little is known about the factors that regulate hair cell genesis and differentiation. The mitogenic effects of insulin and transforming growth factor alpha (TGFalpha) were assayed in vivo in normal and drug-damaged rat inner ear. Tritiated thymidine and autoradiographic techniques were used to identify cells synthesizing DNA. Simultaneous infusion of TGFalpha and insulin directly into the inner ear of adult rats stimulated DNA synthesis in the vestibular sensory receptor epithelium. New supporting cells and putative new hair cells were produced. Infusion of insulin alone or TGFalpha alone failed to stimulate significant DNA synthesis. These results suggest that exogenous growth factors may have utility for therapeutic treatment of hearing and balance disorders in vivo.