We have maintained isolated oligodendrocytes in culture for up to 5 months. Over time cultured cells acquire conspicuous membranous profiles that either emanate from cell processes and perikarya or are apposed to them. These membranous profiles have biochemical and ultrastructural features characteristic of myelin. Because the cells used in this study are post-myelination (cells are isolated from white matter of mature brains), we argue that this is a case of resynthesis of myelin membranes. Because this process takes place in the absence of neurons, we have termed it myelin palingenesis in order to distinguish it from remyelination, a term commonly used to designate reensheathment of axons. Myelin palingenesis may occur seemingly without wrapping and spiraling, but membranes wrapped around oligodendrocyte processes and soma are also observed. Irrespective of the modes of formation, those membranes that are compact have the fine structure of normal myelin. Thus, long-term cultures of mature oligodendrocytes should be a good model system in helping to decipher the molecular and cellular mechanisms involved in myelination and in remyelination.