The effect of right ventricular overloading on body surface isopotential maps (maps) was investigated in 42 patients with the atrial septal defect of the secundum type (ASD) and isolated incomplete right bundle branch block without significant cardiovascular disorders. The patients were divided into two groups according to the presence or absence of right ventricular pressure overloading. Thirty-one patients had right ventricular volume overloading alone, ASD (V), with right ventricular systolic pressure of less than 59mmHg, and 11 patients had right ventricular volume overloading accompanied with pressure overloading, ASD(V+P), with right ventricular systolic pressure equal to or greater than 60mmHg. In ASD (V), effects of RV volume overloading appeared in the positive potentials of the right anterior chest leads from the right midaxillar line to the anterior median line, and in the negative potentials of the left lateral chest leads from the left midclavicular line to the left posterior axillar line. The addition of pressure overloading, observed in ASD (V+P), affected the positive and the negative potentials over a slightly larger area than in ASD(V). For estimating right ventricular pressure with maps semiquantitatively, the relationship between right ventricular systolic pressure and potentials of 87 lead-points utilized in maps were investigated in all 42 patients. Linear regression analysis revealed that the sum of the amplitude of the maximum positive potential in leads D3, D4 and D5 and the amplitude of the maximum negative potential in leads F3, F4 and G3 correlated well with right ventricular systolic pressure (r=0.79, p<0.005). In 20 patients who received surgical correction, postoperation maps were compared with the maps obtained before operation. The patterns were not significantly different between the two maps, but the positive and negative potentials decreased in the late stage of ventricular depolarization. The Breakthrough minimun shifted to the right, with no significant changes in appearance time, after the operation.