To determine the more exact site of origin of ventricular premature beats (VPBs), body surface maps were recorded in 28 patients, aged 15 to 71 years (mean 52) with no cardiac disorders. The site of origin of VPBs was determined from 1) sequential isopotential maps, 2) ventricular activation time (VAT) map and 3) on the basis of maps during pacemaker stimulation at different ventricular areas. To get a three dimensional idea regarding the progression of the ventricular activation front, zero plane was constructed in the human torso by supposing the plane which includes zero potential line on body surface at each instance of ventricular activation. Unipolar lead electrocardiograms were recorded from 85 lead points over the body surface to obtain maps. The site of origin was determined to be from right ventricle in 15 patients, from top of interventricular septum in 10 patients, and from left ventricle in 3 patients. Of 15 patients with right ventricular premature beats, 3 were from outflow region, 4 were from posterobasal region, 6 were from inflow region and 2 were from apical region. Of 3 patients with left ventricular premature beats, 2 were from posterobasal region and 1 was from high lateral region.