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The antioxidative effect of quercetin was exrimined in metmyoglobin-induced oxidation of a fish oil-bile salt emulsion (average diameter of particles; 2.0μm) to evaluate its effectiveness during the digestion of highly oxidizabile oils. The activity of quercetin increased with the lowering of the initial peroxide value (PV) of the oil and its effectiveness was superior to that of α-tocopherol. A synergistic antioxidant effect was observed upon the addition of quercetin and α-tocopherol irrespective of the initial PV of the oils, and quercetin was consumed faster than α-tocopherol. The loss of quercetin was larger than that of α-tocopherol when cumene hydropeioxide and metmyoglobin were mixed in a trimyristin-bile salt emulsion. In an ultrafiltration experiment on emulsitied oil with a membrane filter of 100 nm pore size, the recovery of quercetin in the filtrate was higher than that of α-tocopherol. These data suggest that quercetin was an antioxidant in the digestion of fish oil. The effectiveness seems to come from its distribution in the emulsitied oil, diffrient ffom that of α-tocopherol, and its ability to scavenge radicals generated from the reaction of lipid hydfoperoxides with metmyoglobin.