The central axons of Type I spiral ganglion neurons travel in the auditory nerve and terminate in the cochlear nucleus. The ascending branches of these axons innervate the anteroventral cochlear nucleus and give rise to large axosomatic endings, called the endbulbs of Held, and smaller boutons. This paper reports a study of the endbulbs of Held, stained by horseradish peroxidase and variants of the Golgi method in kittens 2, 5, 10, 20, and 45 days postnatal and adult cats. Endbulbs tend to fall into two extreme groups with some endbulbs having an intermediate appearance; consequently, we have defined three descriptive stages of endbulbs that are conceived of as representing a developmental sequence. One group of endbulbs is found mostly in kittens younger than 10 days postnatal and is similar to the classic description of endbulbs by Ramón y Cajal ('09). The other extreme group of endbulbs is found mostly in adult cats. In these cases, the parent axonal trunk divides into several thick, gnarled branches that in turn branch again, sometimes repeatedly. These branches display irregular varicosities and form a cup-shaped arborization into which the postsynaptic cell body nestles. A chronology of postnatal endbulb development has been inferred from the relative proportions of the different endbulb stages at various ages. Maturation transforms the endbulb of Held from a large, spoon-shaped swelling having many filipodia into an elaborate tree with broad trunks and many smaller branches. Some implications of the proposed developmental sequence are discussed.