Plumbago zeylanica (known as "Chitrak") is a useful Indian medicinal plant. The root of the plant and its constituents are credited with potential therapeutic properties including anti-atherogenic, cardiotonic, hepatoprotective and neuroprotective properties. To examine possible mechanisms of action of P. zeylanica (Chitrak), in relation to its reported beneficial properties, antioxidant effects of the aqueous/alcoholic extracts of root, corresponding to medicinal preparations, and the active ingredient, plumbagin, were studied. Methods used included: ferric reducing/antioxidant power (FRAP), radical scavenging of 1,1-diphenyl-2-picryl hydrazyl (DPPH) and 2,2'-azobis-3-ethylbenzthiazoline-6-sulfonic acid (ABTS), lipid peroxidation in rat liver mitochondria induced by different agents, and estimating phenolic and flavonoid content. In FRAP/DPPH assays, boiled ethanolic extracts were the most effective, while in the ABTS assay boiled aqueous extracts were the most efficient. These extracts also significantly inhibited lipid peroxidation induced by cumene hydroperoxide, ascorbate-Fe(2+) and peroxynitrite and contained high amounts of polyphenols and flavonoids. To examine the mechanisms of action in detail, antioxidant and pulse radiolysis studies with plumbagin were conducted. The hydroxyl (.OH), alkyl peroxyl (CCl(3)OO.), linoleic acid peroxyl (LOO.), and glutathiyl (GS.) radicals generate a phenoxyl radical upon reaction with plumbagin. The bimolecular rate constants were: .OH, 2.03 x 10(9) dm(3)mol(-1)s(-1); CCl(3)OO., 1.1 x 10(9) dm(3)mol(-1)s(-1); LOO., 6.7 x 10(7) dm(3)mol(-1)s(-1); and GS., 8.8 x 10(8) dm(3)mol(-1)s(-1). In conclusion, our studies reveal that extracts of P. zeylanica and its active ingredient plumbagin have significant antioxidant abilities that may possibly explain some of the reported therapeutic effects.