The temporal and spatial distribution of immunocytochemically identified reactive astrocytes is described following a cerebral stab wound in adult rats. Different patterns of reactivity were observed in the cerebral cortex, the corpus callosum and the deep structures of the hemisphere. In the cerebral cortex, the zone of astrocytic reactivity was initially limited to the vicinity of the wound but spread with time to encompass the entire ipsilateral cortex, then regressed; in the deep structures only a spreading phase was observed and this was slower than in the cortex; reactivity in the corpus callosum was slight and always restricted to the immediate vicinity of the lesion. Reactive astrocytes were never observed in the hemisphere contralateral to the lesion. The reorganization of reactive astrocytes in the immediate vicinity of the lesion into a membrana gliae limitans accessoria was also observed. On the basis of these observations, the hypothesis is proposed that astrocytes respond primarily to the mechanical disruption consequent to injury and that the response promotes the restoration of the structural integrity of the lesioned tissue.