Loss or absolute lack of myelin in the CNS results in remarkable compensation at the cellular level. In this study on the natural progression of neuropathology in the CNS in 2 related but distinct long-lived dysmyelinated rats, total lack of myelin was associated with remarkable glial cell proliferation and ineffective myelinating activity throughout life in Long Evans Bouncer (LE-bo) rats; conversely, in Long Evans Shaker (LES) rats, futile myelinating activity ceased when rats were advanced in age. Progressively severe astrogliosis separates individual axons from each other and coincides with widespread, abundant axonal sprouting throughout the life in both rat strains. Severely dysmyelinated Long Evans rats can serve as excellent models to elucidate the cellular and molecular mechanisms of neuroglial compensation to lack or loss of myelin in vivo and to study axonal plasticity in the adult demyelinated CNS.