Optic nerve heads of 21 eyes from 13 rhesus monkeys that had developed papilledema secondary to chronically raised intracranial pressure from balloon implantation in the subarachnoid space were studied by light and electron microscopy. Nine eyes were also examined with the horseradish peroxidase tracer technique. The pathologic changes in the optic nerve head included severe axonal changes and mild interstitial edema. Axonal alteration ranged from axonal swelling to formation of cytoid bodies. Interstitial edema and vascular leakage of horseradish peroxidase were noted in the posterior lamina retinalis, lamina choroidalis and lamina scleralis (prelaminar and lamina cribrosa regions), as well as in the retrolaminar myelinated optic nerve. No intracellular edema of the glial elements was noted. Axonal swelling was so prominent that it appeared to be primarily responsible for the overall swelling of the optic disc observed ophthalmoscopically.