The hippocampal formation of mouse, rat, guinea pig, rabbit, cat, dog, goat and monkey were seen by sulfide silver technique. Intense positive reaction to the this method was found in the stratum radiatum of h(3), h(4) and h(5) of the hippocampus, as well as the stratum multiforme of the fastia dentata. All these areas corespond to the endings of mossy fibers at the apical dendrit of pyramidal cells in the hippocampal formation. No histochemical reaction of zinc was observed in the brains of rat of newborns up to 10 day after birth. The hippocampal formation showed a slight reaction on and after the 11 day of life and the zinc contents of these areas reacted almost normal level about 2 weeks after birth. Histochemical evidence has been observed by the sulfide silver method that the zinc distributed in the region becomes temporarily undemonstrable for a certain brief period (1-3 hours) following administration of dithizone (200 mg/kg). Sections taken at 1.5 hours after administration of dithizone (5 mg/kg) showed positive reaction by the sulfide silver method. During the first three hours following the administration of dithizon (10 mg/kg - 300 mg/kg) the hippocampal formation remains unstained. The distribution of zinc in the hippocampal formation of the rat was changed by circadian rhythm.