The decarboxylated noradrenaline metabolite 3,4-dihydroxymandelic acid [DHMA, 2-(3,4-dihydroxyphenyl)-2-hydroxyacetic acid] occurs in different mammalian tissues, especially in the heart. To elucidate the physiological function of DHMA, the antioxidative and radical scavenging activity was determined by physicochemical and cell-based test systems. In the 2,2-diphenyl-1-picrylhydrazyl assay it shows a 4-fold higher radical scavenging activity compared to the standard antioxidants ascorbic acid, tocopherol, and butylated hydroxytoluene. DHMA is also a very potent superoxide radical scavenger and shows a 5-fold smaller IC(50) value compared to standard ascorbic acid. Again, in most cases the antioxidative power of DHMA against bulk lipid oxidation determined by accelerated autoxidation of oils is much higher than for the standard antioxidants. In soybean oil and squalene a DHMA/alpha-tocopherol mixture (1:1 w/w) shows a synergistic effect. Last but not least, 0.001 and 0.0005% levels of DHMA protect human primary fibroblasts against H(2)O(2)-induced oxidative stress as determined by the 2',7'-dichlorofluorescein assay.