Quantitative measurements of the composition of axonal bundles were made from electron micrographs for six levels within the intraocular portion of the optic nerve head of the rhesus moneky. The proportion of the areas occupied by axonal cytoplasm, glial cytoplasm, mitochondria, and myelin was calculated. Average axonal diameters were determined at each location. Axonal bundles acquired progressively more interaxonal glial tissue as they passed posteriorly from the region of the optic disk toward the optic nerve. Mitochondria were present within axons in increased numbers in the lamina choroidalis and lamina scleralis, and there may be focal expansions and constrictions of axons in the same regions. The overall cross-sectional area of the intraocular portion of the optic nerve expanded posterior to Bruch's membrane in part because of the addition of the interaxonal glial tissue.