A branch of the dorsal interosseous nerve is described, which innervates the dorsal aspect of the wrist joint capsule of the cat. Recordings of the whole wrist joint nerve showed no tonic activity until the wrist was flexed to within 45 degrees of full flexion (about one third of the normal range). There was no tonic activity at full extension or at intermediate postions. 110 single afferents from the wrist joint capsule were isolated from dorsal root filaments. Their responses to movement of the wrist and other mechanical stimuli were examined, and their conduction velocities measured. 31 were slowly adapting, 41 were phasic, 9 were Pacinian corpuscle-like, 7 were weakly activated, and 22 were not activated by the mechanical stimuli which were used. Slowly adapting receptors responded only when the joint was in a flexed position as would be expected from the whole nerve data. Conduction velocities for the 4 classes of responding mechanoreceptors were not significantly different, and fell in the group II range. Fibre diameter histograms of myelinated axons in the wrist joint nerve showed peaks in the group II and group III range. Intravenous injection of succinylcholine showed no increase in whole nerve activity attributable to muscle spindles, and had a negligible effect on identified receptors in the wrist joint capsule.