A new disorder of central myelination has been recognised in male Springer Spaniel pups which is probably inherited in a sex-linked recessive mode. The affected animals were much reduced in weight an size and showed gross generalised tremor, particularly when aroused, at about 10-12 days of age. Affected pups were studied between 1 and 3 months of age. There was severe hypomyelination throughout the CNS which was more marked in the cerebrum and optic nerves than in the spinal cord. The amount of myelin at each location increased with age. Axonal calibre also increased and there was no difference between the axonal diameters of affected and age-matched normal pups. Axons were either naked or surrounded by a disproportionately thin layer of myelin. Myelinated internodes tended to be short and heminodes were frequent. Vacuoles were present adjacent to axons or within glia but there was no evidence of demyelination. Total glial numbers were not reduced and numerous oligodendroglial and astrocytic nuclei identified. Peripheral, cranial and autonomic nerves were myelinated normally. It is suggested that there is an abnormality of oligodendroglial metabolism such that they cannot form and maintain normal myelin. Consequently the radial and longitudinal extensions of their plasma membranes are reduced. The vacuoles may represent a breakdown of defective myelin lipids as suggested in certain murine mutants. This defect of myelination provides a further model in which normal and disordered myelinogenesis can be studied.