We studied the effect of antioxidant enzymes on the loss of integrity of the blood-brain barrier in the optic nerves of strain-13 guinea pigs with chronic experimental allergic encephalomyelitis, a demyelinating disorder with neurologic and histopathologic characteristics similar to multiple sclerosis. Animals with experimental allergic encephalomyelitis received daily intraperitoneal injections of either preservative-free saline (group 1), catalase (group 2), or glutathione peroxidase (group 3) for 2.2 months after the onset of appendicular paralysis. Following intravascular administration, extravascular leakage of horseradish peroxidase was histopathologically graded as mild, moderate, or severe within the optic nerve head and myelinated retrolaminar nerve. Severe extravasation of horseradish peroxidase was exclusive to group 1, in addition to moderate and mild leakage. In groups 2 and 3, leakage of horseradish peroxidase was infrequent, and when detected, it was graded as mild. Detoxification of hydrogen peroxide with catalase and glutathione peroxidase substantially reduced horseradish peroxidase leakage in experimental optic neuritis, suggesting a role for hydrogen peroxide and its reactive by-products in the pathogenesis of increased vascular permeability of the blood-brain barrier in experimental allergic encephalomyelitis.