His bundle electrograms were recorded in three patients with heart block who subsequently died. Serial sections of the conduction system were performed. Two patients had complete block distal to H with wide QRS and P-H intervals of 135 and 100 msec, respectively (normal 80-140 msec). Postmortem (PM) revealed relatively insignificant A-V nodal lesions and major destructive lesions of both bundle branches. The third patient had block proximal to H with left bundle-branch block and H-V prolongation. PM revealed amyloidosis of the A-V nodal approaches and fatty infiltration of the A-V node and His bundle. There were also fibroelastic lesions, with total disruption of the left and moderate involvement of the right bundle branch.The first patient also had sinus pauses as well as delay between what was felt to be posterior internodal tract depolarization and the P wave. PM also revealed arteriolosclerosis, mononuclear cell infiltration, and fibroelastosis of the SA nodal approaches.Block distal to H appeared to reflect total disruption of both bundle branches. Normal P-H intervals appeared to correlate with relatively insignificant A-V nodal lesions. Block proximal to H with bundle-branch block and H-V prolongation reflected multiple conduction lesions. SA block appeared to reflect involvement of the SA nodal approaches.