Papilledema was induced in monkeys by raising the intracranial pressure or by ocular hypotony. Fine structure examination of the swollen discs showed that the extracellular spaces were enlarged in well-perfused areas of the subhyaloid portion of prelaminar optic disc and the adjacent nerve fiber layer of the retina. Axonal swelling was present mostly in areas where there was evidence of inadequate perfusion. It is concluded that the axonal swellings are fixation artifacts, and that fluid enters the axons from an enlarged extracellular space during fixation. The glial cells in the prelaminar optic nerve were not significantly swollen, which suggests that swelling of astrocytes plays no part in the etiology of uncomplicated papilledema. Axons were occasionally distended with myelin figures and mitochondria. The possible mechanism of fluid accumulation in the optic disc is discussed.