Rabbits subjected to facial nerve transection during cortisone acetate treatment display acute depression of regenerative changes in neuronal perikarya and of reactive mitosis in microglial cells. To determine whether these reactions can result in a more severe impairment of neuronal regeneration, chronic experiments have been started for quantitative studies. A comparison with thyronine-treated animals is carried out in order to establish the nature of the action of the two hormones on the central nervous system. This may have some relevance to the prognostication of clinical observations and treatment.