Horizontal and frontal plane QRS loops of patients in sinus rhythm with uniform ventricular extrasystoles were constructed from digitised Frank orthogonal electrocardiograms. In 4 patients ventricular extrasystoles were indistinguishable from right bundle-branch complexes, and in another 4 from left bundle-branch complexes. In 25 patients ventricular extrasystoles showed an initial delay (≧ 20 msec) of the QRS, followed by an R loop, which in 13 patients resembled LBBB complexes, and 12 patients resembled RBBB complexes with an anterior clockwise loop in the horizontal plane. However, the frontal plane loop was often atypical of either RBBB or LBBB. In 51 patients ventricular extrasystoles had an initial delay, which was prolonged into the efferent limb of the QRS loop with acceleration of the afferent limb and/or the QRS loop was directed anteriorly, inferiorly and to the left. Conventional recordings of these extrasystoles usually showed an initial slow upstroke (or downstroke) of the QRS resembling a delta wave. It is suggested that extrasystoles resembling bundle-branch complexes arise close to the main bundle branches but that the other extrasystoles arise at the Purkinje-myocardial junction. Identification and differentiation of ventricular extrasystoles from aberrant conduction is aided by vestorcardiography.