The radical scavenging activity of tenoxicam against hydroxyl (HO.), superoxide (O2.-), and peroxyl (LOO.) radicals, all of them involved in the inflammatory reactions, has been tested in different cell-free systems and by different techniques. Tenoxicam is a good scavenger of both HO. radicals (IC50 = 56.7 microM), as determined by Electron Spin Resonance (ESR) spectroscopy with the spin trapping (5,5-dimethyl-1-pyrroline N-oxide, DMPO) technique, and O2.- radicals generated by the phenazine methosulfate/reduced beta-nicotinamide adenine dinucleotide (PMS/NADH) system. The high reactivity of the drug towards HO. was confirmed by the rate constant of reaction with HO. (k approximately 10(10) M-1s-1), determined by competition kinetic studies with N,N-dimethyl-4-nitrosoaniline. In addition at a microM level (1-5 microM) it dose-dependently prevents the phycoerythrin peroxidation induced by the water-soluble azoinitiator 2,2-azobis (2-amidinopropane) dihydrochloride (ABAP), indicating a quenching effect on aqueous peroxyl radicals. The HO.-entrapping capacity was confirmed in models more close to the in vivo situation: tenoxicam inhibits the HO.-induced depolymerization of hyaluronic acid already at 15 microM and the HO.-driven lipid peroxidation in phosphatidylcholine liposomes (PCL) with an IC50 of 10 microM. In this membrane model it delays at 1-10 microM level the decomposition of phosphatidylcholine hydroperoxides to short-chain alkenals (markers: total carbonyl functions as 2,4-dinitrophenylhydrazones and conjugated dienes). The high susceptibility of the drug to HO. attack is also demonstrated by its extensive degradation (HPLC studies) when irradiated with HO. radicals. The antioxidant component of tenoxicam evidenced in this study sheds some light on the hitherto undefined mechanism of the antiinflammatory action of the drug.