Two types of mechanoreceptor have been found in the articular capsule of the knee joint of the domestic cat--Ruffini corpuscles and Pacinian corpuscles. Ruffini corpuscles are situated in the stratum fibrosum and consist of 2 to 6 cylinders. Each cylinder is made up of an afferent axon (diameter 3-4 micrometer), its swellings and terminal processes, Schwann cells enveloping the nerve swellings and terminal processes, endoneural connective tissue and a perineural capsule. The perineural capsule is incomplete in Ruffini corpuscles. The Pacinian corpuscles are 20 to 40 micrometer wide and 150-250 micrometer long. They are situated in groups of up to five at the boundary between the stratum synoviale and the stratum fibrosum. The afferent axon is myelinated (diameter 3-5 micrometer). Its terminal portion is inside the inner bulb which is formed of modified Schwann cells. Each corpuscle is enveloped by a perineural capsule (4-8 layers). The ultrastructure of the Pacinian corpuscles is compared with the ultrastructure of the skin receptors in the cat.