Lidocaine (L) in still considered the drug of choice in the treatment of life threatening ventricular arrhythmias in the setting of acute myocardial infarction (A.M.I.): this is mainly due to its proved efficacy and high therapeutic index. Since however L has well defined electrophysiologic properties (class 1 B) and does not seem to be effective in all of these patients we compared its antiarrhythmic efficacy with the one of amiodarone (A), an antiarrhythmic agent provided of electrophysiologic properties quite different from L (class 4) and usually well tolerated. Twenty-five consecutive patients with A.M.I. without haemodynamic consequences, were randomly assigned to L (bolus of 1 mg/kg followed by an infusion of 10 mg/min for 20 minutes and thereafter of 1,5 mg/min) or A (bolus of 5 mg/kg and an eventual repeat dose of 150 mg followed by an infusion of 1,8 g/24 h) The baseline arrhythmia was classified as Lown class 2 ore more in all the patients. The ventricular arrhythmias were completely relieved in 47% of the patients assigned to L and in 60% of those treated with A (p = N.S.); a minor efficacy was found in 40% of the patients of both groups; in two cases ventricular fibrillation occurred after the acute infusion of L. The antiarrhythmic effectiveness kept fit with both drugs over a 24 hours period. A prolongation of QTc interval was found to occur both after the bolus and 24 hours of treatment with A. In addition A provoked a slight decrease of systolic blood pressure. No important side effects were observed with both drugs regimens.(ABSTRACT TRUNCATED AT 250 WORDS)