An embryologic study of rat hippocampal formation was performed. By the silver sulfide method for light microscopy, the same positive reaction as in adult rats was observed in 20-day rats in the same regions: the border between stratum radiatum and stratum pyramidale of h(3) and h(4), hilus fascia dentata(h5) and stratum multiforme of Gyrus dentatus. These regions corresponded to the site of mossy fiber endings. In the silver sulfide method modified for electron microscopy, deposits of silver grains were observed in mossy fiber endings of 8-day rats, and the morphology of mossy fiber endings of 14-day rats was similar to that of adult rats. These results suggest that these areas begin to mature in 8 days and complete maturation in 20 days after birth.