The ultrastructure and distribution patterns of sensory nerve endings in the dorsal knee joint capsules of the beagle dog (Canis familiaris) have been investigated using light and electron microscopy. Each dorsal knee joint capsule was divided into four quadrants, cut into small pieces and then processed for electron microscopy. Free nerve endings and corpuscular nerve endings (Ruffini and lamellated corpuscles) were found. They were most frequently observed in the medial-proximal quadrant of the dorsal joint capsule. All nerve endings were found to be situated within or adjacent to the fibrous layer of the capsule. No nerve endings were found within the synovial layer. Free nerve endings were usually situated at the border between the fibrous layer and the synovial layer near blood vessels. Their associated afferent axon was myelinated (1.5-2.5 microns in diameter) or non-myelinated (0.3-1.5 microns in diameter). Ruffini corpuscles were found in the fibrous layer and within the dorsal ligamentous apparatus. Each Ruffini corpuscle was surrounded by a multilayered perineural capsule which was usually incompletely developed. The perineural capsule is the continuation of the perineurium of the afferent axon and gives a cylindrical form to the corpuscles. Ruffini corpuscles were present as single, cylindrical structures (small corpuscles) or as aggregates of these cylinders (large corpuscles). Both varieties consist of terminal nerve endings surrounded by collagen fibres which pass through the opened ends of the cylinders. The diameter and length of the small Ruffini corpuscles were 80 microns and 400 microns, as compared to 200 microns and 800 microns for the large aggregated forms. The supplying afferent axons of both types were 4-5 microns in diameter. Two types of small lamellated corpuscles could be observed in the fibrous layer: very small corpuscles, 55 microns long, 25 microns wide and medium corpuscles, 100 microns long, 40 microns wide. Each consists of an inner core of terminal Schwann cells, a nerve terminal and a perineural capsule. Some lamellated corpuscles had two inner cores and two nerve terminals. The diameter of the afferent axon was approximately 6 microns. Vater-Pacini corpuscles were not found in the dorsal knee joint capsule of the dog.