The retrograde response of HRP-labelled intercostal motoneurons was compared in adult (1-2 years) and aging (10-15 years) cats, 64-68 days following crush of intercostal nerves or following nerve transection with proximal ligation. A comparison of the neuroglial response to these two lesions was also made. In both adult and aging cats, 64-68 days following nerve crush, most labelled motoneurons had a normal appearance. In contrast, 64-68 days following nerve transection and ligation the Nissl bodies of labelled motoneurons lacked the highly ordered ultrastructure characteristic of normal and control motoneurons. No axotomy-induced neuronal loss was found in aging cats. A three-fold increase in numbers of microglial cells was quantified in the ipsilateral ventral horn of aging cats following nerve transection and ligation. This increase was not seen following nerve crush in aging cats, nor following either type of nerve injury in adult cats. Numbers of astroglia and oligodendroglia were unaffected by axotomy in adult and aging animals.