Oligodendroglia in the "Shiverer" spinal cord at 2, 4, 8 and 18 weeks of age were investigated by electron microscopy, with special reference to the process of myelin formation. The predominant changes observed were vacuolization of the cytoplasm during 2 and especially 4 weeks of age, indicating initial steps in the formation of myelin sheaths, although their total volumes was very small and their lamellae were incomplete. Polymorphic vacuoles and vesicles with various electrondense contents appeared to originate from the Golgi complex and the rough-surfaced endoplasmic reticulum (r-ER), since such vacuoles happened to be continuous with the Golgi complex, and the Golgi complex disappeared concomitantly with the appearance of great numbers of vacuoles. The r-ER was also fragmented together with the occurrence of vesicles. After 8 weeks of age, the number of myelinated nerve fibers reached the plateau of the increasing curve, while the number of myelin lamellae successively increased until the sheaths were completed. At these stages the oligodendroglia containing vacuoles gradually disappeared, although numerous oligodendroglia possessing a nucleus with peripheral, densely clumped chromatin were present. These dark oligodendroglia contained a Golgi complex, r-ER with small, single cisternae and microtubules of irregular thickness, different from those found in the control oligodendroglia. At 18 weeks of age polymorphic vacuoles had disappeared in most oligodendroglia.