Timm's sulfide silver method was employed in studies of zinc localization in the hippocampal formation of the rat. Both the technique reported by Haug and that of Ibata and Otsuka have been used to perform Timm's method. Various problems with Timm's method, including fixation, duration of immersion in Timm's developing solution, temperature and illumination were evaluated in order to obtain the best possible results.The technique described in this communication afforded the following results. The silver particles were deposited preferentially in the synaptic vesicles of the mossy fiber terminals. None were seen in the mitochondria or in the dendritic spine. No silver granules were noted either in the pyramidal cells of the hippocampus or in the granular cells of the dendate gyrus. The preferntial localization of zinc in the mossy fiber ending is discussed.