In immature rats active macrophages were frequently seen projecting into the subarachnoid space from the surface of the leptomeninges. They also occurred between the layers of the pia and within the nerve roots. They were most frequent during the first two weeks after birth, which is a period of rapid neural growth and myelination in ventral roots. In contrast, they were much fewer at later stages. The ultrastructural characteristics of these cells are described. It is suggested that these cells take part in tissue growth and remodelling by the removal of material which degenerates or becomes redundant during development. For example, they may ingest effete leptomeningeal cells or fragments of them. Those within the ventral roots may phagocytose abnormal Schwann cells, or the myelin of sheaths which have failed to develop normally. It is also suggested that macrophages may be involved in the excavation of the subarachnoid space. Another possible function in which they may be involved is the ingestion of material, possibly of a protein nature, from the cerebrospinal fluid.