It has occasionally been claimed that multiple sclerosis (MS) may be due to a chronic mercury intoxication, e.g. from mercury liberated from dental fillings. Therefore, the present communication compares the mercury content assayed by neutron activation in 8 macroscopically normal areas (frontal lobe) of MS autopsy brains with those of 8 control samples. No significant differences could be traced between the two groups concerning total mercury. However, the lipid-soluble mercury (preferably methyl mercury) expressed per cell unit (DNA) was found significantly decreased in MS. These data may be explained either by a wash-out of lipid soluble mercury due to break-down of the blood-brain barrier in MS or to abnormalities in methylation processes probably related to the vitamin B12 metabolism in MS.