Male weanling mice (Biobreeding Laboratories) exposed to the drug Cuprizone (biscyclohexanone, oxaldihydrazone) in the diet for periods of 6 weeks and longer, consistently showed almost complete demyelination of the superior cerebellar peduncle. The demyelination was primary and followed degeneration of oligodendrocytes and their processes, whereas axons remained intact. After formation of myelinic vacuoles and removal of myelin by macrophages and astrocytes, the axons became invested with astroglial processes. As part of the glial response to demyelination, numerous reactive or immature cells appeared, some of which were identified as being either astrocytic or oligodendrocytic in nature. Some mature oligodendrocytes survived. When allowed to recover on a normal diet, remyelination began within a week, and progressed until all axons were myelinated. The mechanism of remyelination appeared similar to the spiral wrapping mechanism seen in normal development. The myelinating cell in all cases was the mature oligodendrocyte. Sources for these oligodendrocytes include residual surviving oligodendrocytes, differentiation of immature forms, and possibly the perineuronal satellite cell. The sheaths eventually reached a thickness approximately half that of normal development, with a disturbed relationship between myelin thickness and axon diameter. A visual impression of shortened internodal length was obtained. It is concluded that the Cuprizone model is an excellent situation in which to study the cellular mechanisms of demyelination and remyelination.