Lipid peroxidation mediated by hydroxyl radicals which are generated during myocardial ischermia has suggested as a possible mechanism of ischemic myocardial damage. Recently, it has been reported that anti-arrhythmic action of lidocaine, a local anesthetic, is attributed to its "membrane-stabilizing" properties through scavenging free radicals, thus, inhibiting lipid peroxidation. Aldehyde oxidase and xanthine oxidase which catalyze the oxidation of many purine, pyrimidine and pteridine derivatives are known as free radical generating systems. In this experiment, we studied the effect of lidocaine and procainamide on the hepatic aldehyde and xanthine oxidase activity and antioxidative activities. It was found that lidocaine and procainamide inhibited both NADPH-dependent and independent lipid peroxidation. Both of tested compounds were found to be ineffective in inhibiting xanthine oxidase. Lidocaine and procainamide, however, inhibited aldehyde oxidase activity in vitro as well as in vivo. Based on the above results, lidocaine and procainamide could be employed as a therapeutic agent for aldehyde oxidaserelated disease.d disease.