The nervous system of the rats with methyl mercury intoxication was studied by a recently developed electron microscopic X-ray microanalytical technique. A mercury element was detected at the lysosome-like dense bodies of the peripheral nerves and of the cerebellum, especially prominent at the dense bodies of the collapsing cells. Mercury was also suggested at the dilated endoplasmic reticulum of Schwann cells. It may be apparent that mercury itself practically penetrates into the cells of the nervous system and directly destructs them. Except for the lysosome-like dense bodies and occasional endoplasmic reticulum, however, the precise localization and distribution of minimal amounts of mercury element within the cells were difficult to evaluate by this method, because a peak for mercury was overlapped occasionally with that of sulphur when they were so scanty in quantity.