Abstract. While interventricular septal mass amounts to nearly 30% of the whole ventricle, the ECG effect of septal myocardial infarction (MI) has not yet been fully understood. In order to clarify electrical effects of septal MI, a computer simulation study was performed with use of three dimensional computer model (Wei-Harumi). The heart model consisted of 50.000 units of cells was mounted in the homogeneous torso. The action potential was assigned to each of the units and the resulting electromotive force was calculated to reconstruct the surface ECG. Septal MI was represented by non-excitable units corresponding to necrotic area. The action potential duration in the border zone of necrotic area, where blood supply was obtained from both left and right coronary arteries, was lengthened by 30 to 90 msec compared with the normal zone. The results showed small r waves in leads V1 and V2, and the QS pattern in midprecordial leads. T waves in V1, V2 were shifted to the negative side. Conclusions; It is suggested that the QS pattern frequently seen in anteroseptal infarction depends largely on the septal lesion and abnormal repolarization force from the border zone causes negative precordial T waves.