The application of magnetic resonance imaging, spectroscopy and electrophysiological techniques to the study of multiple sclerosis has enhanced our understanding of the mechanism of relapse and remission. The earliest detectable event in the development of a new lesion is an increase in permeability of the blood-brain barrier associated with inflammation. Demyelination occurs early in the inflammatory phase. Both processes contribute to conduction block and functional loss. When the inflammation subsides, edema resolves and conduction is restored probably as a result of the expansion of sodium channels into the demyelinated axon. Remyelination is not essential to remission. Wallerian degeneration may be an important factor contributing to irrecoverable deficit.