Superoxide (O2-.) is a highly toxic free radical that may be an important component of pulmonary O2 toxicity. The primary defense against this free radical is superoxide dismutase. Rats were exposed to aerosolized superoxide dismutase, and it failed to modify either the time course or the cumulative toxicity of 100 per cent O2. Because the aerosolized enzyme can be expected to be delivered only to the extracellular space of the lung, it is suggested that the primary site of production and of damage due to O2-induced free radicals must be within the intracellular space.