The distribution of heavy metals were studied in rat brain by light and electron microscopy with the sulfide silver method. Reaction products by this method were found under light microscopy in olfactory bulb, the hippocampal formation, subiculum, and amygdaloid nucleus belonging to the limbic system, and in caudate nucleus, lenticular nucleus, red nucleus and substantia nigra belonging to the extrapyramidal tracts. These products were also observed in neocortex of cerebral cortex and paraventricular nucleus. These reaction positive-materials were not found in nerve cell bodies in all these area but found in neuropil containing many synapses. Electron microscopic observation of the hippocampal formation showed that these materials were distributed in synaptic vesicles of the mossy fibers. From these results the morphological and functional relationship between these products (heavy metals) and synapses, especially the ending of presynaptic fibers, were discussed.