Applying different stereological techniques, the total ependymal volume in the spinal cord of mice was estimated to be 83 x 10(6) microns cubed, the number of cells to be 163,000 and the mean ependymal cell volume to be 510 microns cubed. Compared to choroid plexus cells in the third ventricle, the ependymal cells in the spinal cord contained a smaller mitochondrial volume (9.8% versus 4.6% of cell volume) and less rough endoplasmic reticulum (2.1% versus 0.4%). These findings indicate that the metabolic activity of the ependyma in the spinal cord is lower than that in the choroid plexus. Compared to liver and exocrine pancreatic cells, ependymal cells in both locations must be considered to have a rather low metabolic activity.