The Ruffini organ is an arborized axon ending categorized as a low-threshold stretch receptor. We have previously shown that the lingual periodontal ligament of rat incisors is densely innervated with Ruffini endings. In the present study, fine structures in the surface of the periodontal Ruffini endings and their topographical relationship with the surrounding collagen fibers were observed by a combination of scanning and transmission electron microscopy to analyze the mechanism of the stretch reception. The entire length of the branches of the Ruffini endings, excepting their terminal portions, corresponded well with those depicted by previous investigators in the following points: (1) their cylindrical appearance covered by Schwann cell processes; (2) the presence of numerous axon fingers protruding through gaps in the Schwann sheath and; (3) their isolation from collagen fibers by multilayered basal lamina. On the other hand, tips of the axon branches-together with their Schwann sheaths-became attenuated and projected into tight bundles of collagen, indicating their susceptibility to mechanical deformations of the surrounding tissue. Margins of the axon terminals were conspicuously ruffled with long tongue-like projections of Schwann cells. The Schwann cell tongues twined around collagen bundles in their distal portions, and associated closely with fine axon projections in their proximal portions, suggesting their involvement in the mechanical transmission of stimuli to axon terminals.