&NA;This study evaluates the effect of superoxide dismutase, a scavenger of oxygen free radicals, on peritumoral and cold injury edema. Vasogenic brain edema was produced in 39 cats by a standardized cortical freezing lesion. Peritumoral edema was produced by the transplantation of VX2 carcinoma cells into the brain of New Zealand White rabbits. Detection of superoxide radicals was studied by topical application of nitroblue tetrazolium to the cortical injury and by incubation of the VX2 carcinoma cells. The aniamls were treated with free superoxide dismutase (SOD) and polyethylene glycol superoxide dismutase (PEG‐SOD). The following groups were studied: 1) control, 2) untreated tumor group, 3) tumor group treated with PEG‐SOD, 4) a group treated with PEG‐SOD before injury, and 5) a group treated with free SOD after injury. Brain edema was evaluated by measurement of specific gravity and planimetry study of the spread of Evans blue dye. Preliminary data indicate that superoxide radicals are present in the brain after cold injury and in the cytoplasm and nucleus of VX2 carcinoma cells. Free SOD and PEG‐SOD had no beneficial effect upon the vasogenic brain edema produced in either model. It is concluded that intracellular uptake of SOD may be one of the factors necessary for an effect upon either form of edema. (Neurosurgery24:679‐685, 1989)