a b s t r a c tPlectranthus barbatus Andrews (Lamiaceae) is a popular medicinal plant used to treat gastrointestinal and hepatic ailments. In this work, we assessed the antioxidant activity of the aqueous extract of P. barbatus leaves on Fe 2+ -citrate-mediated membrane lipid peroxidation in isolated rat liver mitochondria, as well in non-mitochondrial systems: DPPH reduction, Å OH scavenging activity, and iron chelation by prevention of formation of the Fe 2+ -bathophenanthroline disulfonic acid (BPS) complex. Within all the tested concentrations (15-75 lg/ml), P. barbatus extract presented significant free radical-scavenging activity (IC 50 = 35.8 ± 0.27 lg/ml in the DPPH Å assay and IC 50 = 69.1 ± 0.73 lg/ml in the Å OH assay) and chelated iron (IC 50 = 30.4 ± 3.31 lg/ml). Over the same concentration range, the plant extract protected mitochondria against Fe 2+ /citrate-mediated swelling and malondialdehyde production, a property that persisted even after simulation of its passage through the digestive tract. These effects could be attributed to the phenolic compounds, nepetoidin -caffeic acid esters, present in the extract. Therefore, P. barbatus extract prevents mitochondrial membrane lipid peroxidation, probably by chelation of iron, revealing potential applicability as a therapeutic source of molecules against diseases involving mitochondrial iron overload.