Immunohistochemical methods were used to study the status and organization of astrocytes, oligodendroglia and microglia in fetal cerebral cortical tissue grafted on to the dorsal surface of the midbrain in newborn host rats. Grafts were examined 1-6 months posttransplantation. All grafts contained large numbers of hypertrophied, intensely glial fibrillary acidic protein-positive astrocytes. Microglia were also activated, displaying slightly increased levels of OX-42 immunoreactivity. The grafts consisted of lobules of gray matter which were separated by bands of myelinated fibres associated with large numbers of Rip-positive oligodendroglia. These glial cells had a relatively normal morphology. The density of astrocytes and microglia was reduced in these white matter-like regions. In association with chronic changes in glial reactivity, transplants also expressed increased levels of chondroitin sulphate proteoglycans (CS-56 antibody). The observed changes in glial cell phenotype and extracellular matrix in cortical transplants are likely to affect neuronal physiology and connectivity in a number of ways, and highlight the importance of studying both glia and neurons in order to gain a more comprehensive picture of the long-term functional potential of fetal brain grafts. (C) 1997 Elsevier Science Ireland Ltd.