The number of neuroglial cells in selected fiber tracts of 90-day-old quaking and normal mice was determined by a combination of light and electron microscopy. Oligodendrocytes of quaking mice are normal in number in the anterior commissure and corticospinal tract (in the cervical spinal cord) but are increased two- to fourfold in the optic nerve and the fasciculi cuneatus and gracilis (in the cervical spinal cord). The nuclei and perikarya are normal in size or smaller than normal. Those tracts with the greatest hyperplasia of oligodendrocytes also have the greatest content of myelin, suggesting that cell number influences content of myelin. However, the volume of myelin per oligodendrocyte also varies, between 2 and 11% of normal, in the different tracts of the mutant. The hyperplasia of oligodendrocytes in quaking mice may arise as compensation for their decreased production of myelin and reflect a normal plasticity in the processes of myelination. If so, the mutant may be a useful system for study of the regulation of myelogenesis.