Concomitant with the proliferation of satellite microglial cells occurring in the process of motor neuron regeneration, an astrocytic hypertrophy is also seen. A remarkable increase of glial fibrillary acidic protein (GFAP) immunoreactivity is demonstrated in astrocytes of the facial nucleus within a few days following nerve transection. The increase of GFAP antigenicity is associated with an increased appearance of glial filaments and astrocytic processes. We suggest that resident protoplasmic astrocytes become involved in retrograde changes in facial motor neurons and transform into reactive astrocytes. They are of the fibrous type and highly positive for GFAP.