In humans, hair cell loss often leads to hearing and balance impairments. Hair cell replacement is vigorous and spontaneous in avians and nonmammalian vertebrates. In mammals, in contrast, it occurs at a very low rate, or not at all, presumably because of a very low level of supporting cell proliferation following injury. Heregulin (HRG), a member of the epidermal growth factor (EGF) family of growth factors, is reported to be a potent mitogen for neonatal rat vestibular sensory epithelium, but its effects in adults are unknown. We report here that HRG-alpha stimulates cell proliferation in organotypic cultures of neonatal, but not adult, mouse utricular sensory epithelia. Our findings support the idea that the proliferative capabilities of the adult mammalian vestibular sensory epithelia differ significantly from that seen in neonatal animals. Immunohistochemistry reveals that HRG-binding receptors (erbBs 2-4) and erbB1 are widely expressed in vestibular and auditory sensory epithelia in neonatal and adult mouse inner ear. The distribution of erbBs in the neonatal and adult mouse ear is consistent with the EGF receptor/ligand family regulating diverse cellular processes in the inner ear, including cell proliferation and differentiation.