Discrepancies between previous light microscopic studies on the localization of basic fibroblast growth factor (bFGF) in neuroglias and neurons of the normal rat brain prompted us to investigate, by electron microscopy, the subcellular localization of bFGF-like immunoreactivity in neuroglias and neurons of the trigeminal motor and mesencephalic nuclei. Immunostaining intensity differed from astrocyte to astrocyte; in astrocytes labeled heavily with bFGF antiserum, the precise subcellular location of immunoreaction deposits was difficult to determine, whereas mildly labeled astrocytes contained reaction products in subcellular regions apart from gliofilaments, mitochondria and Golgi apparatus. A comparison of immunostained sections with negative control ones at the light and electron microscopic levels revealed that astrocyte nuclei occasionally showed bFGF immunoreactivity. Immunoreactive astrocyte processes were also found in close apposition to blood vessels. bFGF was detected mainly in intracellular structures close to free ribosomes and the endoplasmic reticulum of immunoreactive oligodendrocytes and neurons; microglias rarely showed immunostaining. The nuclei of the cells with bFGF contained immunoreaction deposits of varying intensity, mainly in the euchromatin and rarely in the heterochromatin. Occasionally, bFGF of neuroglial origin accumlated in the vicinity of the interface between neurons and neuroglias. The extracellular matrix was not immunoreactive in any of the areas examined. These findings suggest that certain populations of astrocytes and oligodendrocytes as well as neurons in the normal brain contain bFGF-like substances.