The new technique of nuclear magnetic resonance imaging (NMR) has been found to have particular value in the study of the evolution of the plaque of multiple sclerosis. Particularly when combined with gadolinium enhancement, the method not only shows very dramatically the waxing and waning of the plaque with time, it also demonstrates with remarkable clarity the important role of changes in vascular permeability in the pathological process. In this Annotation the ability of this technique to throw new light on the process of plaque formation and evaluation is critically assessed. In addition, the role of changing fluid content of the extracellular spaces of the CNS in influencing interpretation of the more conventional clinical and electrophysiological findings is discussed. While the method of NMR analysis does not yet show us how the plaque is initiated, it is suggested that future studies with these new techniques in the living subject may well lead us to rational therapeutic approaches based on pathogenetic mechanisms.