A technique for examination of the conducting system of the heart is described. A block of tissue embracing the ostium of the coronary sinus, the pars membranacea, the septal leaflet of the tricuspid valve, and appropriate amounts of interatrial and interventricular septum is flattened and fixed in a Kaiserling I solution. Blocks are subsequently cleared in methyl salicylate and trimmed. Sections are cut from the blocks after paraffin embedding beginning from the endocardial surfaces of the right heart chambers. The sections are mounted and stained on 35 mm unperforated leader film and covered with an acrylic preservative. For examination of the conducting system many fewer sections are required than with previous techniques.