The mechanism of abnormal motion of the interventricular septum (IVS) in the altered ventricular activation process was investigated using M-mode echocardiograms (ECHO). Subjects consisted of 37 patients in whom artificial pacemakers was implanted because of sick sinus syndrome in 14 and atrioventricular block in 23. Standard 12-lead electrocardiograms (ECGs) were classified in three groups based on the polarity of the main deflections of the QRS complexes in leads V1 and V6; the pattern was designated as the LBBB type if the main QRS deflections were negative in V1 and positive in V6; as the S type if they were negative in both V1 and V6; and as the RBBB type if positive in V1 and negative in V6. In the LBBB type the excitation was assumed to begin in the RV and to propagate across the IVS to the LV. The S type was considered an expression of activation process which starts in the apex and is propagated radially toward the base with nearly simultaneous activation of the RV and the LV. These assumptions were confirmed by the study of contraction sequences using phase image analysis of cardiac blood pool scintigrams. The RBBB type was thought to reflect delayed activation of the RV. ECHO showed two characteristic patterns of IVS motion in the patients studied. During preejection the IVS moved rapidly posteriorly, then anteriorly, forming a hump. During ejection, anterior displacement or akinesis of the IVS (paradox) was observed. A hump was frequently observed in any types of ECG.(ABSTRACT TRUNCATED AT 250 WORDS)