Postnatal development of the mouse cerebellar mossy fiber endings was studied with electron microscope.1. The endings are recognized around the 5th postnatal day and attain the adult pattern during the 4th postnatal week.2. Special attention was paid to the electron dense granules (200-400A) in the immature endings. The amount of dense granules, as well as the number of the endings with them, gradually increase with maturation of the animal. They reach their peak during the 10th to 13th postnatal day, diminish thereafter and disappear after 3 weeks postnatally.3. On the basis of several experiments these granules can be identified as glycogen, though their staining properties do not meet the generally accepted criteria.4. As to the significance of glycogen which are found in the developing endings, some possibilities were considered and discussed.