A permanent gross demonstration of dog cardiac conduction tissue was attempted unsuccessfully using standard glycogen and fat stains, experimentally induced lipogenesis, iodine, and the iodides of fluorescein, silver, and lead. Staining with insoluble sulfides was also unsuccessful. Permanent gross differentiation of the tissue can be accomplished with palladium iodide. The best fixative for minimum tissue distortion which stillpermits good differentiation is neutralized formol-saline. The opened and fixed heart is dipped in alcohol for 15 minutes prior to staining, then placed in an iodine-potassium iodide solution for two to five minutes. It is then rinsed in running water for one minute and transferred to a solution containing PdCl2 and HCl and left for ten minutes. Following the staining reaction, a differentiation lasting several hours is necessary. By this method, the entire sub-endocardial ramificationof the specialized tissue can be permanently demonstrated. Alternatively, the above process may be replaced by painting the endocardium with the various solutions, the results being equivalent but more localized.