Abstract Mexiletine was given to 59 patients with acute or chronic ventricular arrhythmias. Efficacy was assessed by long-term tape-recording of electrocardiograms with computer analysis. Intravenous mexiletine successfully suppressed acute ventricular arrhythmias in 31 of 43 patients, with partial control in a further 9. After intravenous bolus injections initial high rates of infusion were required to maintain therapeutic plasma concentrations. Cardiovascular toxicity occurred in 6, and severe non-cardiac toxicity in 9. Oral mexiletine was well absorbed and suppressed non-acute ventricular arrhythmias in 12 of 16 patients, with partial control in 3. Therapeutic plasma concentrations were in the range 0·5-2·0 μg. per ml., while toxicity was usually associated with levels above 3·0 μg. per ml. The mean plasma half-life was 18·7 hours in patients and 10·2 hours in healthy volunteers. Although intravenous mexiletine was effective in the treatment of acute ventricular arrhythmias, its main potential appears to be as a long-term oral antiarrhythmic agent.