The mechanisms that control the production and differentiation of glial cells during development are difficult to unravel because of displacement of precursor cells from their sites of origin to their permanent location. The two main neuroglial cells in the rat spinal cord are oligodendrocytes and astrocytes. Considerable evidence supports the view that oligodendrocytes in the spinal cord are derived from a region of the ventral ventricular zone (VZ). Some astrocytes, at least, may arise from radial glia. In this study a 5-Bromo-2'-deoxyuridine (BrdU) incorporation assay was used to identify proliferating cells and examine the location of proliferating glial precursor cells in the embryonic spinal cord at different times post BrdU incorporation. In this way the migration of proliferating cells into spinal cord white matter could be followed. At E14, most of the proliferating cells in the periventricular region were located dorsally and these cells were probably proliferating neuronal precursors. At E16 and E18, the majority of the proliferating cells in the periventricular region were located ventrally. In the white matter the number of proliferating cells increased as the animals increased in age and much of this proliferation occurred locally. BrdU labelling showed that glial precursor cells migrate from their ventral and dorsal VZ birth sites to peripheral regions of the cord. Furthermore although the majority of proliferating cells in the spinal cord at E16 and E18 were located in the ventral periventricular region, some proliferating cells remained in the dorsal VZ region of the cord.