Staining with the Griffonia simplicifolia II lectin (GSL II), a marker for adult rat oligodendrocytes, was studied in rat brain at embryonic (E) days E14, E16, E18, and E20. At E14, only two regions of the CNS showed labeling--the hippocampal primordium and the ventromedial floor of the mesencephalic aqueduct and fourth ventricle. Labeling in these areas was confined to cells with bipolar processes which spanned from the ventricular lumen to the pial surface. At E16, GSL II+ radial glia were present in the hippocampus, septal area, cerebral peduncle, thalamus, tegmentum, and throughout much of the midbrain. At E16, E18, and E20, GSL II strongly labeled two midline radial glial raphes--dorsal to the mesencephalic aqueduct and ventral to the mesencephalic aqueduct and fourth ventricle. Comparison of GSL II lectin labeling with nestin immunostaining, using Rat-401, showed that GSL II labeled a distinct population of radial cells different from nestin-positive radial glia. GSL II also labeled a group of neurons at E16 and E18 in the hypothalamus and basal striatum. We suggest that GSL II+ radial glia represent a population of oligodendroglial precursors present in the embryonic brain, and that GSL II reactivity could serve as a differentiation marker for cells committed to the oligodendroglial lineage. This work supports the possibility that some oligodendroglia, like astrocytes, are derived from radial glial precursors.