Body surface isopotential maps were produced by computer processing of the 85 electrocardiograms obtained from the entire thorax of 28 patients with complete or incomplete right bundle branch block (RBBB). We divided the map patterns into the following 3 groups. Type I map pattern (10 cases): at the early stage of QRS, the maximum was located in the left chest. It shifted to the left from the normal position; at the instant of 44 msec, on the average, after the onset of QRS breakthrough minimum appeared over the left chest. Its appearance was delayed and its site shifted to the left as compared with the normal; at the late stage, the positive zone covered extensively the right chest and the right back; terminally, the maximum was positioned along the right parasternum. Type II map pattern (13 cases): at the early stage of QRS, the maximum was in the left chest as in Type I; breakthrough minimum appeared at 38 msec on the average, later than in the normal, but the site of breakthrough minimum varied from the left chest as in Type I to the midsternal region as in the normal; at the late stage, the positive zone covered the upper part of the right chest and the right back, less extensively than in Type I; the terminal maximum was in the upper sternal region. Type III map pattern (5 cases): the map pattern passed normally until the late stage, but thereafter a small positive zone survived over the upper sternal region. In Type I the delayed activation was presumed to occur all over the right ventricle, in Type II mainly over the smaller area of the right anterior free wall, and in Type III over the localized area of the outflow tract. Patients with complete RBBB showed Type I pattern. Patients with incomplete RBBB showed Type II or Type III pattern, although electrocardiograms failed to differentiate Type II patients from Type III patients. These findings suggest that the electrocardiographic pattern of incomplete RBBB probably arises from the various mechanisms.