Fifty-four patients with a symptomatic ventricular arrhythmia underwent two 24-hour electrocardiographic recordings, respectively before and after two to three weeks treatment with mexiletine, at a dose of 200 mg three times a day. According to the reduction in the number of premature beats, the effectiveness of the treatment was similar for patients with heart disease (group A), and for those suffering from an idiopathic arrhythmia (group B). A reduction of more than 80% was observed in 48.1% of the patients, and it was ranging between 50 and 80% in 13% of them. An increase in the number of premature beats was observed in 14.8%. According to the Lown's grading system, the shift from malignant (IV-V) to benign (I-III) grades was significant in group A only. The side effects justified the discontinuation of the treatment for 22.8% of an initial group of 70 patients, but were benign and totally reversible. Thus, mexiletine appears to be effective in the treatment of ventricular arrhythmias, with a relatively high incidence of benign and reversible side effects.