To determine the effect of several oxidation stages of autoxidized methyl linoleate (AOML) on glutathione peroxidase (GSH-Px), several experiments were carried out. In the first experiment, GSH-Px was prepared from gastrointestinal tract and liver which were excised from mice, and the effects of AOML rich in hydroperoxides (HP) and that rich in the decomposition products of HP on the activity of GSH-Px were compared. The results indicate that GSH-Px indiscriminately metabolized both types of AOML without strict recognition of the specificity of the substrate. In the second experiment, it was noted that the inhibitory rate of GSH-Px by in vitro incubation with various AOML closely correlated with POV of the sample esters, but did not correspond with the toxicity revealed by intraperitoneal injection or intragastric feeding. In the next experiment, mice were given AOML with a low vitamin E diet for 45 days, and an increase of GSH-Px activity in gastrointestinal tract in proportion to the POV of the administered oil occurred, but the level in liver remained unchanged. On the contrary, the increase of GSH-Px activity by intraperitoneal injection of AOML in mice was marked in liver, but not in gastrointestinal tract. From these results, most of the orally administered AOML seemed to be reduced in the mucosa of the gastrointestinal tract. However, the marked increase of fluorescence in the lipid fraction of heart and kidney by oral administration of AOML suggested that the damage caused by it was not limited to gastrointestinal tract but was spread through the whole body.