Myelin proteolipid protein (PLP), the major myelin protein in the CNS, has been thought to function in myelin assembly. Thus, mutations within the gene coding for PLP (Plp) cause hypomyelination, such as the jimpy phenotype in mice and Pelizaeus-Merzbacher disease in humans. However, these mutants often exhibit premature death of oligodendrocytes, which form CNS myelin. To elucidate the functional roles of Plp gene products in the maturation and/or survival of oligodendrocytes, we produced transgenic mice overexpressing the Plp gene by introducing extra wild-type mouse Plp genes. Surprisingly, transgenic mice bearing 4 more Plp genes exhibited dysmyelination in the CNS, whereas those with 2 more Plp genes showed normal myelination at an early age (3 weeks after birth), but later developed demyelination. Overexpression of the Plp gene resulted in arrested maturation of oligodendrocytes, and the severity of arrest was dependent on the extent of overexpression. Overexpression also led to oligodendrocyte cell death, apparently caused by abnormal swelling of the Golgi apparatus. Thus, tight regulation of Plp gene expression is necessary for normal oligodendrocyte differentiation and survival, and its overexpression can be the cause of both dys- and demyelination.