Quantification of glial cells, axonal size and myelin thickness and volume were carried out in selected areas of the three funiculi of the cervical spinal cord and the optic nerve of 'shaking pups' and normal littermates at 4 and 8 weeks of age. There was a marked reduction of oligodendrocytes in the affected pups with many of these cells having distended rough endoplasmic reticulum. Oligodendrocyte death was not noticeable. Astrocyte numbers were similar in both normal and affected pups. Axonal diameters were not reduced in the affected pups and there was no apparent correlation between myelination and axonal size in these animals. Total myelin volume and thickness were greatly reduced in the 'shaking pups.' Impaired stem cell division together with metabolic disturbance of oligodendrocytes are considered to be the main causes of the hypomyelination in this mutant.