Guinea-pigs, which exhibit a similar metabolism to that of man insofar as they are incapable of synthesizing ascorbic acid, were poisoned with Gramoxon containing the LD50 or LD100 of paraquat. 2. A study was made of how the paraquat poisoning affected previously investigated tissue parameters, the superoxide dismutase, catalase and glutathione peroxidase activities and the lipid peroxidation. 3. In accordance with earlier results, the paraquat influences these parameters in different ways in all the tissues examined, depending on its concentration. 4. These parameters comprise primarily affected systems as regards paraquat toxicity.