Sulfonic acids of quercetin and morin as well as their ferrous and cupric complexes were synthetized and investigated. Sulfonic derivatives of quercetin were much weaker inhibitors of soybean lipoxygenase than quercetin itself. Morin and its derivatives were inactive. Antioxidant properties of quercetin derivatives were in the same range as for quercetin. Most of the investigated compounds stimulate cyclooxygenase when 100 microM of arachidonic acid is used as a substrate. Ferrous complex of quercetin 5'-sulfonic acid was an inhibitor of this enzyme.