Linoleic and arachidonic acids are competing substrates for 5-1ipoxygenase from barley. When these two substrates are added simultaneously, arachidonic acid acts as a competitive inhibitor of linoleic acid oxidation with K i of 20 μM, the same value as the Michaelis constant for arachidonate oxygenation by this enzyme (22 ± 3 μM). Linoleic acid hydroperoxide accumulated in the reaction mixture does not inhibit the enzymatic process, while arachidonic acid hydroperoxy product (5-hydroperoxy-6,8,11,14-eicosatetraenoic acid) inhibits it with very low K i equal to 0.5 μM.