Ultrastructural studies of sensory endings in monkey posterior medial knee joint capsule were undertaken. Three distinct sensory nerve endings have been identified: free nerve endings, Ruffini corpuscles, and Pacinian corpuscles. The free nerve endings are present in all layers of the joint capsule excluding the synovium. Two types of Ruffini corpuscles have been found in the fibrous layer. The first type is characterized by a thin perineurial capsule, the second type by a thicker perineurial capsule and extensive intra-capsular space. Both types of Ruffini corpuscles are innervated by approximately one to four myelinated axons which lose their sheaths as they course through the corpuscle. They terminate on collagen fiber bundles as distinct swellings with spiny membrane projections that are covered by a thin basal lamina. These terminals contain abundant mitochondria, agranular vesicles, and irregularly arranged neurofilaments and neurotubules. Two types of Pacinian corpuscles were occasionally observed. The first was a small, typically laminated structure with an inner core at the layer between the synovium and the fibrous layer and between the fibrous layer and muscle/ligament; larger Vater-Pacinian corpuscles were noted only at the boundary between the fibrous layer and the muscle/ligament layer.