Progressively growing intracranial space-taking lesions were simulated in 32 rhesus monkeys by balloons introduced into the subarachnoid space of the temporal region. Optic disc edema (ODE) first appeared at the lower pole, then the upper pole, then the nasal part, and last the temporal part of the disc; severity of edema generally followed sult, most severe at the lower pole (P less than .005). Fluorescein fundus angiography showed that swelling of the optic disc preceded the vascular changes associated with ODE. Raised intracranial pressure for 24 hours, or less, could cause ODE. The atrophic part of the optic disc did not develop ODE. The studies indicate that swelling of the optic disc is the first sign of raised intracranial pressure and is due to swelling of the nerve fibers in the optic disc; the various associated vascular changes are secondary.