Using protocols that incorporated double blind examination of animals sensitized to CNS antigen, we confirmed and amplified earlier findings of the complete suppression of EAE in rodents by hyperbaric oxygen. The effects of O2 were related to the gas pressure and duration of treatment. The development of paralytic disease was prevented for 34 days after sensitization (the longest interval studied). Compressed air or normobaric O2 administered under similar conditions did not modify the course of illness. Within 7 to 10 days after the discontinuance of oxygen therapy the majority of treated guinea pigs developed typical signs of EAE with characteristic lesions in the CNS. The relapses occurred sooner in the Lewis rat. The development of the delayed hypersensitivity reaction to myelin basic protein and to tuberculin is also suppressed by O2 therapy indicating that its effects upon autoimmune encephalomyelitis involves fundamental alterations of the cellular components of the immune response, some or all of which are reversible.