Two postmortem case of multiple sclerosis treated by sterotactic operations for the intention shaking of limbs, trunk, and head, and for the action myoclonus are analyzed to determine the location of the substrate of myoclonic and ballistic movements, the location of the coagulations for relief of these movements, and whether fresh demyelinating foci are elicited by intracerebral interventions. In the first case of a clinically typical multiple sclerosis, the foci responsible for the severe action myoclonus and intention ataxia of the trunk are demyelinations in the right and left red nucleus resulting in nerve cell damage and loss and an almost complete destruction of myelinated fibers. The restricted foci in the white matter of the cerebellum which do not involve the cerebellar nuclei are not extensive enough or old enough to be the cause of the action myoclonus but may, perhaps, sustain the pathogenesis. - In the second case of cerebral palsy and combined multiple sclerosis (detected post mortem), the combination of the severe damage of putamen and caudate nucleus by status marmoratus and the extensive nerve cell and fiber damage due to demyelinating foci in the substantia nigra are probably the substrate of the jactitation and intention myoclonus of the left limbs. The stereotactic coagulation of the dentatothalamic and pallidothalamic fibers in the base of V. o.p. and V.o.a. at the point where they pass through the zona incerta (location confirmed post mortem) resulted in a nearly complete relief of hyperkinetic movements. In the first case, fresh demyelinating foci are present in both hemispheres with stereotactic interventions; these foci are located, amongother places, around the coagulation and the electrode track. In the second case, post mortem serial brain sections demonstrate that stereotactic operations even in subacute multiple sclerosis can be carried out without eliciting any exacerbation of demyelination foci. Therefore, the danger exists that stereotactic intervention in cases of multiple sclerosis may precipitate fresh demyelinating foci. As our clinical experience [Riechert and Richter, 1972a, b] indicates, however, this occurred in markedly less than 10% of the cases.