A double-blind, placebo-controlled trial comparing the antiarrhythmic effects of lidocaine (given intravenously as a bolus injection of 100 mg followed by an infusion of 2 mg min-1) and propafenone (given as a bolus of 105 mg followed by 300 mg orally every 8 h) was conducted in the first 24 h following acute myocardial infarction. Analysis of ventricular arrhythmias was carried out by Holter recordings. The three treatment groups, propafenone (36 patients), lidocaine (28 patients), and placebo (25 patients), did not differ with respect to age, gender, prevalence of previous infarction, delay from the onset of pain to hospitalization, clinical features on entry (the patients with heart failure or malignant arrhythmias were excluded), site of acute myocardial infarction, or CPK peak. A decrease in the number of ventricular premature beats was noted with lidocaine, but was not statistically significant. The analysis of the first 8 h showed trends suggesting that only lidocaine could suppress complex arrhythmias, couplets and ventricular tachycardia. The drugs were well tolerated. Mean plasma concentrations of propafenone and lidocaine were 517 +/- 464 ng ml-1 and 3.84 +/- 1.10 mg l-1, respectively. In conclusion, this study does not favour the use of propafenone as an alternative to lidocaine therapy during the acute phase of myocardial infarction.