An electrocardiographic atlas of ventricular tachycardias was produced by pacing 27 epicardial sections of the heart and the mitral papillary muscles to simulate focal ventricular arrhythmias and simultaneously recording their 12 lead electrocardiographic appearances. One hundred and twenty nine patients undergoing cardiac surgery were studied. In five patients all 27 epicardial sites were paced at operation and in 124 individual sections were paced postoperatively with temporary epicardial wires and the electrocardiograms analysed in terms of frontal and horizontal plan QRS axis, maximum limb lead QRS amplitude, and QRS duration. Each ventricular region paced produced a distinctive 12 lead electrocardiographic pattern. Simulated right ventricular arrhythmias had either inferior frontal plane QRS axes (from the anterobasal region) or superior frontal plane QRS axes (from the apical and posterior right ventricular sections). Horizontal plane QRS axes were directed leftwards, with some posterior shift in the anteroapical regions. Simulated arrhythmias from the base of the left ventricle (anteriorly and laterally) had inferior frontal plane QRS and anterorightward horizontal plane QRS axes. Left ventricular arrhythmias with a superior frontal plane QRS axis were readily distinguished by their horizontal plane QRS axes: posterorightwards from the anterior and anterorightwards from the posterior left ventricular sections. Standard errors of the paced QRS axes for the various epicardial sections paced postoperatively ranged from 3.0 degrees to 6.0 degrees using the frontal plane axis. The electrocardiogram was most accurate in localising ventricular arrhythmias from the anterior left ventricle and least accurate for those arising from the inferior right ventricle. The appearance of the paced electrocardiograms was slightly modified by underlying disease such as myocardial infarction and left ventricular hypertrophy. This atlas may be useful in comparing the localisation of ventricular tachycardia with the site of underlying cardiac disease and may facilitate mapping in patients with refractory ventricular tachycardia requiring ablation (either surgical or by high energy impulses).