The postnatal development of innervation in the lingual periodontal ligament of rat incisors was investigated by immunohistochemistry using an antibody against protein gene product 9.5 (PGP 9.5), a neuron-specific protein. Immunostaining with the antibody enabled the clear demonstration of chronological alteration in the distribution and maturation of nerve terminals after birth. At 1 day after birth, thick nerve fibers showing PGP 9.5-immunoreactivity first appeared in the loose connective tissue corresponding to the future periodontal ligament, without forming an apparent terminal structure. At 4 days, two portions became distinguishable in the lingual periodontal ligament: alveolus- and tooth-related parts being comparable to those in the adult ligament. The immunoreactive nerve fibers increased in density and appeared to extend more incisally. Those restricted to the alveolus-related part showed ramifications with a small number of expanded or bulbous portions, and terminated among the periodontal fibers. Characteristic dendritic and expanded terminals, similar to Ruffini endings in the adult ligaments, began to occur in the lingual periodontal ligament at 7 days after birth and gradually increased in number until day 11 when the incisal edges emerged in the oral cavity. At approximately 15 days after birth, nerve endings possessing morphological features similar to Ruffini endings in adult rats were recognizable in the entire length of the periodontal ligament. After the eruption of first molars, the periodontal Ruffini endings drastically increased in number, and by the time the occlusion between the first molars was established, became identical in distribution and density to those seen in the lingual periodontal ligament of adult rat incisors. These findings suggest that the mechanical stimuli due to tooth eruption and occlusion might be a prerequisite for the final differentiation and maturation of the periodontal Ruffini endings.