The volume and the morphology of brain white matter as well as the number and the size of glial fibrillary acidic protein (GFAP)-immunoreactive astrocytes were investigated in 6-month-old spontaneously hypertensive rats (SHR) and age-matched normotensive Wistar-Kyoto (WKY) rats. The volume of frontal and occipital cortex and of hippocampus was decreased in SHR in comparison with normotensive rats, whereas the volume of neostriatum was unchanged. A remarkable decrease of the volume of internal capsule and striosomes, a moderate reduction of that of corpus callosum and no changes of the volume of external capsule and of white matter of hippocampus were also observed in SHR. In SHR the number of astrocytes was higher in the frontal and occipital cortex and in the white matter of the CA1 and CA3 subfields of the hippocampus, but not in the corpus callosum or in the grey matter of the CA1 and CA3 subfields. Staining for myelin did not reveal alterations in single fibre sheath morphology. These findings indicate the occurrence of changes of forebrain white matter in SHR, consisting in the reduction of it without qualitative modifications of myelinated fibres. The development of gliosis apparently not related with changes of volume of white matter was also found.