The pathological alterations in the central nervous system (CNS) of spontaneously epileptic rats (SER) were reviewed in comparison with those of zitter (Zi) and tremor (Tm) rats which are original neurological mutants for SER. The main pathological findings were vacuolation and hypomyelination in the CNS. Vacuolation was prominent in SER and Tm, especially in the brain stem and thalamus. Ultrastructurally, vacuoles consisted mainly of swollen astrocytic processes and enlargement of extracellular space, as well as occasional enlargement of periaxonal spaces. Throughout the observation period, there was no inflammatory cell infiltrations nor phagocytotic cells. Vacuolation seemed to be mainly derived from tremor mutation.On the other hand, hypomyelination seemed to be related to zitter mutation, based on the chronological observation of early myelination in Zi from 1st to 28th days of age. The characteristic findings accompanied with hypomyelination were the presence of aberrant myelin sheaths in addition to a decreased number of myelinated fibers and increased number of radial components whose real significance has been still unknown. Zi revealed another characteristic alteration in oligodendrocytes, which was membranous abnormality associated with nuclear membrane or other intracellular organelles, such as mitochondria or endoplasmic reticulum. The structures of myelin sheath in these three kinds of mutants seemed to be quite normal and the expression of main structural proteins, such as myelin basic protein (MBP), proteolipid protein (PLP) and myelin associated glycoprotein (MAG) were also seemed to be normal.According to the pathological findings obtained to date, the two different genetic abnormalities (zitter and tremor mutations) interact and synergistically reinforce each other both in clinically and neuropathologically in this double mutant (SER) . To elucidate the etiology and pathogenesis of clinical symptoms and pathological alterations, further neuropathological investigations cooperated with other research fileds, such as genetics, biochemistry, pharmacology, electrophysiology and so on will be required.