Perivascular cells in the rat brain are an immunophenotypically defined group of cells which can be identified by their expression of the ED2 antigen. The present study investigates the role of perivascular cells as scavengers in the perivascular spaces of the rat brain and the relationship of these cells to microglia, macrophages, pericytes and smooth muscle cells. Particulate matter (Indian ink) was injected selectively into the perivascular spaces of the left caudoputamen of 59 rats. Animals were killed by cardiac perfusion of formalin or glutaraldehyde 2 h-2 years after ink injection. Cerebral hemispheres were examined histologically and immuno-cytochemically using the ED2 antibody for perivascular cells, ED1 for microglia and macrophages and OX-6 directed against la antigen [major histocompatibility complex (MHC) class II]. ED2⁺ perivascular cells ingested Indian ink in the perivascular spaces and expressed MHC class II antigen. Reactive microglia and macrophages in the perivascular parenchyma expressed ED1, but no ED2⁺ cells were seen outside the perivascular spaces. Transmission electron microscopy distinguished perivascular cells, which ingested carbon particles, from pericytes, which did not. The results of this study suggest that perivascular cells remain distinct from pericytes, microglia and macrophages and that they play a major role as scavengers in the perivascular spaces of the rat brain. This role reflects the improtance of perivascular spaces as drainage pathways for soluble and insoluble material from the brain.