SUMMARYThe dehydrogenase activities of rat lung, brain and liver were measured before and after exposure of rats to oxygen at 60 p.s.i. gauge pressure (OHP). The rate of methylene blue coupled oxidation of lung tissue was progressively decreased by OHP with an approximately 50 p.c. decline in activity following one hour exposure of animals to OHP. Pentobarbital sodium or cysteamine, administered prior to exposure to OHP, prevented the inhibition of enzyme activity. Death by anoxia had no effect on the dehydrogenase activity of lung tissue.No decrease in the rate of methylene blue coupled oxidation occurred in brain or liver tissue of rats exposed to OHP.