The Long Evans shaker (les) rat is a recently identified CNS myelin mutant with an autosomal recessive mode of inheritance. Although scattered myelin sheaths are present in some areas of the CNS, most notably the ventral spinal cord in the young neonatal rat, this myelin is gradually lost, and 8-12 weeks little myelin is present throughout the CNS. Despite this severe myelin deficiency, some mutants may live beyond one year of age. Rare, thin myelin sheaths that are present early in development lack myelin basic protein (MBP) and on ultrastructural examination are poorly compacted and lack a major dense line. Many oligodendrocytes develop an accumulation of vesicles and membranous bodies, but no abnormal cell death is observed. In the optic nerve, cell kinetic studies show an increase in proliferation at early time points in les, while total glial cell counts are also increased in les from 2 months of age. In situ hybridization studies demonstrate that the numbers of mature oligodendrocytes are similar to controls early in life and increase with time compared to controls. There is both a progressive astrocyte hypertrophy and microgliosis. While les has a mutation in the myelin basic protein (mbp) gene, it is dissimilar in both genotype and phenotype to the previously described mbp mouse mutants, shiverer (shi) and shiverer(mld). Unlike shi and its allele, where myelin increases with time and oligodendrocytes become ultrastructurally normal, les oligodendrocytes are permanently disabled, continue to demonstrate cytoplasmic abnormalities, and fail to produce myelin beyond the first weeks of life.