This study was designed to detect reflection of epicardial breakthrough to body surface isopotential maps by recording epicardial and body surface maps simultaneously, and to estimate changes in epicardial breakthrough associated with complete and incomplete blocks induced by compression of the main stem of the right bundle branch of trans-sectioning of the lateral branches. In the control, epicardial breakthrough appearing on the right ventricular surface was well detected on the body surface maps as a bend of isopotential lines localized at the mid sternum at 11.5 +/- 1.6 msec (n = 5) after QRS initiation. At complete block immediately after compression of the main stem, the localized bend shifted inferiorly to the left at 17.8 +/- 1.8 msec (n = 5) after QRS initiation, suggesting appearance of the left ventricular epicardial breakthrough. With progression of recovery from the compression, in addition to epicardial breakthrough on the left ventricle, the breakthrough on the right ventricle became detectable again with a delay of 5 msec, and then the former was faded away as time progressed. After trans-sectioning of the lateral branches, sequential changes in the body surface maps were almost the same as in complete block of the main stem but they lapsed about 43 msec (n = 5) shorter in comparison with the complete block. In conclusion, detection of the localized bend of the isopotential lines on the body surface can provide diagnosis of the site and degree of the right bundle branch block in detail.