Chronic progressive compression of the optic nerve in the cat was produced by an inflatable silicon rubber balloon implanted in the orbit. Within the first week the predominant pathological change was demyelination. Both partially and completely demyelinated fibres were seen at all stages of the experiments but by the fifth week some axons had been remyelinated by oligodendrocytes despite the continued presence of the balloon. The myelin sheaths of the new internodes were abnormally thin and short, and many showed an atypical paranodal organization. There was evidence of breakdown of some of the new internodes. The pathophysiology of optic nerve compression in man is reviewed.