An attempt was made to label injured cardiac muscle cells by exposing them to two electron-opaque tracers, ruthenium red and lanthanum nitrate. To do this, false tendons of sheep hearts containing strands of Purkinje fibers were sectioned, allowed to heal, and then exposed to the tracer during fixation. After this treatment, a group of cells near the cut end were found to be labelled intracellularly with the tracers while the remaining cells in the strand were unlabelled. For comparison, several false tendons were fixed briefly in glutaraldehyde before being cut and then exposed to the tracer. With lanthanum, the results were similar to those obtained when the cells had been damaged prior to fixation. However, when ruthenium red was used as the tracer, it penetrated much further into the cellular strand, its intensity gradually diminishing with distance from the cut end. This finding of apparent dye-coupling in fixed tissue was surprising since it has been suggested that glutaraldehyde fixation converts all communicating junctions to be uncoupled state. Dye-coupling of fixed tissue with ruthenium red as a tracer was seen also in frog atrial trabeculae. Gap junctions between injured (and presumably uncoupled) sheep heart Purkinje cells were compared to gap junctions between uninjured control cells in thin sections. No difference was detected.