Studies on nuclear volume have shown that it is an indication of the state of differentiation of cells. This study provides evidence indicating increasing nuclear volume during cell maturation. Using unbiased stereological techniques, nuclear volume of both proliferating and non-proliferating glial cells was analysed in the developing spinal cord. Proliferating glial precursor cells were identified using a 5-bromo-2'-deoxyuridine (BrdU) incorporation assay. The nuclear volume of BrdU-labelled cells and unlabelled cells was determined in both periventricular regions and the white matter of the cord at different embryonic ages. In the periventricular region BrdU-labelled nuclei were smaller than unlabelled nuclei at all ages examined. These labelled cells represent dividing undifferentiated progenitors. The unlabelled neighbouring cells with larger nuclei represent a more differentiated population. In the white matter BrdU-labelled nuclei were of similar volume to the unlabelled nuclei. Both of these groups represent glial precursor cells that have migrated from deeper regions and are at similar stages of differentiation, perhaps with different proliferative potential. These findings indicate that the nuclear volume of early glial cells increases as these cells migrate and differentiate.