Fifty-one patients with symptomatic ventricular tachycardia who failed control on current anti-arrhythmics were studied. Seventy-four percent had ischemic heart disease and 81% had congestive heart failure. Patients underwent serial 24 Holter recordings and radionuclide ventriculography before, during dose titration and during long-term mexiletine therapy. Twenty-eight patients (55%) were successfully controlled. Of these, 17 (33%) remained controlled greater than or equal to 1 year. Early and late side effects were common but benign and included mostly gastric pain and nausea. Twenty-eight patients underwent radionuclide ventriculography before and during mexiletine therapy: there was no significant difference in heart rate, blood pressure, left ventricular ejection fraction, stroke volume and end-diastolic volumes before and during mexiletine. Left ventricular ejection fraction was 21.4 +/- 2.2%, (SD) and 21.3 +/- 2.2% (SD) before and during mexiletine respectively. Digoxin blood levels measured in 15 patients were not significantly changed by mexiletine. In conclusion, mexiletine is effective and safe in many patients with intractable ventricular tachycardia. It has no significant hemodynamic effects even in patients with congestive heart failure nor does it affect digoxin blood levels. Its usefulness is limited by a high incidence of gastric intolerance.