The healing response of the periodontal tissues to polytetrafluorethylene was examined in the beagle dog. At midroot level, by way of a mucosal incision and a surgically created fenestration in the facial alveolar bone, a cylinder of polytetrafluorethylene 2 mm. in diameter was fitted tightly into a cavity preparation and cut even with the root surface. Histologic observations were made after 57 and 91 days of healing. In sites with optimal healing (minimal or no inflammation present) a thin layer of connective tissue with capillaries had proliferated into the narrow space between the implant and the cavity walls, and new cementum had formed on the walls and floors of the preparations. Bone replacement occurred in the peripheral parts of the fenestrations and in several instances a complete or nearly complete covering of bone had developed at the outer surface of the implant as well. Osseous tissue often contacted the polymer. Inflammatory infiltrate, concentrated at the cavosurface angles, was observed in sites with the poorest healing. This was associated with microscopic pulpal exposure and/or irritants in the interface between the polymer and the cavity walls. The implant material per se did not appear to produce an inflammatory response or impede the development of cementum and bone. Observations confirm the inertness of polytetrafluorethylene and suggest its use as a covering for reimplanted teeth.