A body surface isopotential map was recorded in 30 cases of ASD and 3 cases of ECD with right bundle branch block in ECG, then analyzed in comparison with results obtained by cardiac catetherization and two dimensional echocardiograms. 1) In all cases of right bundle branch block, breakthrough minimum appeared later and shifted to the left compared with the normal. 2) Breakthrough minimum of ASD and ECD appeared progressively later associated with Qp/Qs ratio and left to right intracardiac shunt ratio obtained with cardiac catetherization and RVDd with echocardiogram. Breakthrough minimum of ASD and ECD moved to the left accompanied by increases in mean RV pressure, mean PA pressure and RVDd. 3) R'max voltage, the maximum positive potential in the late stage of ventricular depolarization on the right precordial area, increased and correlated well with the mean RV pressure and left to right shunt ratio increment. 4) In the case of ASD, breakthrough minimum appeared later and shifted to the left along with age.