Intermittent exposure to 40 percent oxygen prolongs rat survival in 100 percent oxygen.

Prior intermittent exposure to 40 percent O2 protected rats against fatal O2 toxicity. Protection was not absolute, however, for the death of most rats in 100 percent O2 was not prevented but delayed considerably. Furthermore, those rats which did survive exposure to 100 percent O2 for long periods by virtue of prior-adaptation to increased O2 concentrations had pathologic changes in the lungs typical of chronic O2 toxicity. The very early demise of some rats indicated that 1 of 12 animals is hypersenstiive to high concentrations of inspired O2.