Lipid peroxidation was induced in rat heart mitochondria with FeSO4 and the inhibitory effects of various anthraquinone derivatives were compared. Oxygen consumption and malondialdehyde formation were used to quantitate the amount of lipid peroxidation. Emodin [2], alizarin [13], and alizarin complexone [14] significantly inhibited lipid peroxidation; their potency as inhibitors of lipid peroxidation was higher than that of alpha-tocopherol. Structure-activity analysis showed that two hydroxyl groups arranged in either the ortho- or meta-position in the C ring of the anthraquinone nucleus are required for such derivatives to inhibit lipid peroxidation. The diphenyl-p-picrylhydrazyl test showed that alizarin [13] and alizarin complexone [14] are free-radical scavengers while emodin [2] is not. The mechanism for emodin [2] to inhibit lipid peroxidation is most likely due to inhibition on the propagation of lipid peroxyl radicals in the mitochondrial membrane.