The morphology, intracapsular distribution and postnatal development of the Pacinian corpuscle which is one of the sensory nerve endings of the mouse knee joint, was examined by immunohistochemistry for S-100 protein, and by light and electron microscopy. Pacinian corpuscles existed only in the mouse knee joint on the flexor side, and were observed near the insertion of the capsule at the meniscus, at the insertion of the cruciate ligaments, and in the adipose tissue of the joint capsule. An average of 6-7 Pacinian corpuscles were found in one knee joint. The Pacinian corpuscle consisted of a central axon which was surrounded by approximately 20 thin lamellae of inner core cells and 10 layers of perineural capsular cells, and was enveloped by connective tissue capsule. This basic structure was not notably different from that in other experimental animals (cat, rat, etc.). In observation of the postnatal development from 0 to 7 days of age, only sporadic presence of the fasiculus of unmyelinated nerve fibers was observed in the joint capsule. At 9 days of age, a structure, which is considered to be the original form of the Pacinian corpuscle, was observed. This consisted of a central axon surrounded by irregular cytoplasmic processes of Schwann cells which had basal lamina. At 2 weeks of age, the inner core layers around the axon were formed, making up the basic structure of the Pacinian corpuscle. At 3 weeks of age, the numbers of inner core and perineural capsular layers had increased, and structure was closed to the adult morphology of the Pacinian corpuscle. By the 5 weeks of age, the caveolae and coated vesicles, which are characteristically present in the cytoplasm of the lamellar cells of the Pacinian corpuscle, had increased, and the basal lamina was completed. The Pacinian corpuscle, at this point, had become completed in structure and size. The S-100 protein first became positive in the inner core at 3 weeks of age, but negative in the perineural capsule during the entire period. Thus the inner core originated from the Schwann cells and the perineural capsular cells from the connective tissue. The findings suspected that the Pacinian corpuscles of the mouse knee joint start functioning around 3 weeks of age.