In order to elucidate the mechanism of dentin sensitivity, the ultrastructure, distribution and organization of nerve fibers in the pulpodentinal border zone in the teeth of young human subjects were investigated by means of silver impregnation and electron microscopy. The nerve fibers in this zone were classified into four types by the location of their terminals and pattern of their ramification: the marginal pulpal nerve fibers, the simple predentinal nerve fibers, the complex predentinal nerve fibers and the dentinal nerve fibers. The nerve fibers reached no further than 100 microns from the odontoblast-predentinal border. The nerve fibers terminated exclusively as free endings, and they were thought to conduct the sense of pain from the corresponding zone. The endings of the predentinal or dentinal nerve fibers were mostly located adjacent to the odontoblastic processes, and this appeared to be a reasonable position for these endings to actively receive the changes in the shape of the processes. The odontoblastic process and the nerve ending associated with it can be considered to be functionally a mechanoreceptive complex. In this sense these free nerve endings might be placed in a group different from the free nerve endings in the inner pulp. The mechanoreceptive complex probably plays a central role in the mechanism of dentin sensitivity. It is suggested that stimuli to dentin first produce the deformation or movement of the odontoblastic processes; these mechanical changes are transmitted to the nerve endings, and the dentin sensitivity occurs.