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Arnica (Heterotheca inuloides) is a widely used medicinal plant in México; it has been recognized as anti-inflammatory, analgesic, cytotoxic, scavenger of superoxide anion and also as a preventive of lipid peroxidation. In vivo studies have demonstrated a hepatoprotective action of the methanolic extract of this plant as well as of quercetin, one of its main components, and the evidence obtained pointed out to an antioxidant mechanism. In this work, we focused on the free radical scavenging capacity of acetonic and methanolic extracts of H. inuloides in comparison with reference compounds. The two extracts were 2-12 times more effective (IC50, microg/mL) than the reference compounds to cope with the following radicals or molecules tested: 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS(+)), 2,2-diphenyl-1-picrylhydrazyl (DPPH), peroxynitrite (ONOO(-)), superoxide (O2(-)), singlet oxygen ((1)O(2)), hypochlorous acid (HOCl), hydrogen peroxide (H2O2), hydroxyl (OH). Additionally, five secondary metabolites isolated from the methanolic extract displayed potent concentration-dependent antioxidant effects against reactive oxygen species produced in vitro (IC50 values in the range of 0.018-4.31mg/mL). d-Chiro-inositol showed the higher antioxidant effect against O2(-), H2O2 and OH while spinasterol and quercetin were the most active against (1)O(2) and ONOO(-), respectively. The antioxidant properties of the extracts and metabolites tested partially support the wide use of this plant in traditional medicine., (Copyright 2010 Elsevier Ireland Ltd. All rights reserved.)