The proliferative activity in the hypoglossal nucleus following hypoglossal nerve injury has been studied with the 3H-thymidine analogue, bromodeoxyuridine (BrdUrd). BrdUrd was injected into cisterna magna of the rat brain 2 h prior to killing and subsequently visualised with immunofluorescence. The peak of BrdUrd labelling in the hypoglossal nucleus occurred at 2 d following nerve transection and a lower level at 4 and 7 d postoperatively. BrdUrd labelled cells were also found outside the anatomical boundaries of the hypoglossal nucleus, conceivably mainly corresponding to the dendritic extension of the axotomised neurons. It is therefore concluded that microglial cells are activated in relation to the entire intramedullary portion of the axotomised neurons and not only in the immediate vicinity of the perikarya. Double labelling experiments with specific markers for astrocytes, oligodendrocytes and microglial cells showed that only microglial cells were BrdUrd positive at all postoperative survival times examined. It is therefore concluded that microglial cells are the only glial cell type which proliferate in the hypoglossal nucleus following peripheral nerve injury.