Using rubber-dam isolation and an aseptic and standardized endodontic technique, root canal therapy was performed on the right and left fourth mandibular premolars in four dogs. Each tooth was obturated with N2 paste containing 6.5 percent 14C paraformaldehyde. In each dog at 1 hour, 1, 7, 14, and 28 days, blood was drawn and biopsy specimens were taken of the adjacent lymph nodes, liver, and kidney. At 28 days, the animals were killed, and the experimental and adjacent tissues were processes for light microscopy. Histologic examination demonstrated that particles of the paste placed in the root canal were present in pulp, periapical, and periodontal tissues remote from the original site in the root canal. In addition, 14C-labeled paraformaldehyde which was contained in the N2 paste was found in blood, regional lymph nodes, kidney, and liver. The amount of radioactivity in all the body organs decreased with time.