The inhibitory effect of 2"-O-glycosyl isovitexin (2"-O-GIV), isolated from young barley leaves, on glyoxal formation from the oxidative degradation of three fatty acid ethyl esters was measured. Ethyl linoleate, ethyl linolenate and ethyl arachidonate were oxidized by Fenton's reagent. Glyoxal formed from these fatty acid esters was analysed by gas chromatography after it had been derivatized to quinoxaline with 1,2-phenylenediamine. The comparative study was performed using alpha-tocopherol. Generally, alpha-tocopherol exhibited a greater inhibitory effect at lower levels, whereas 2"-O-GIV showed a greater effect than alpha-tocopherol at higher levels. 2"-O-GIV was more effective than alpha-tocopherol towards fatty acid esters with high numbers of double bonds. 2"-O-GIV exhibited a dose-response effect but alpha-tocopherol did not. Maximum inhibition of 82% was obtained from oxidation of ethyl arachidonate at 8 mumol 2"-O-GIV, whereas maximum inhibition of 77% was observed from oxidation of ethyl arachidonate at 0.25 mumol alpha-tocopherol.