The hippocamal formation of rat brain was studied by the silver sulfide method and was also observed by electron microscopy after the administration of dithizone, alloxan and oxine, respectively. In normal rat, the reaction products were found in the mossy fibers of granular cells of gyrus dentatus which ended to the dendrites of pyramydal cells present in h3, h4 and h5 area of the hippocampus. By electron microscopic observation, the mossy fiber endings of normal brain were filled with synaptic vesicles, and most of them ended to the dendritic spines of pyramidal cells. Zinc shown by silver sulfide method were not detected in the hippocampal formation from 1 to 3 hours after the administration of dithizone, and at the same time, the number of synaptic vesicles of the mossy fiber endings was decreased and the dendritic spines became to be smooth. Alloxan did not any effects of the amount of zinc and the number of synaptic vesicles in mossy fibers, while both of them decreased temporarily at 5 to 15 minutes after the administration of oxine. From these results, the correlation between zinc and synaptic vesicles in mossy fibers, and the mechanism of these drugs' action to zinc content and synaptic vesicles were discussed.