The innervation of the shoulder joint of the rat was investigated. Nerve origin was assessed by injection of a neuronal tracer (WGA-HRP) into the shoulder joint cavity and calcitonin gene-related peptide (CGRP), which is known to be present in some sensory neurons, was detected immunohistochemically with an anti-CGRP antibody. In the ipsilateral sympathetic and dorsal root ganglia, 133-312 and 12-55 nerve cell bodies were respectively labeled by injection of the tracer. In the sympathetic ganglia, 83% of all labeled cells were found in the stellate ganglion and 17% in the superior cervical ganglion. In the dorsal root ganglia, 75% of the labeled cells were found in C4 and the neighboring ganglia (C4-C5), while the rest were observed in C6-8 and T3. This suggested that the origin of sensory innervation for the shoulder joint was mainly in the mid-cervical cord. CGRP-immunoreactive fibers were found in the synovial capsule of the shoulder joint. These fibers were fine and resembled type 4 axons as classified by Brodal, i.e., nerve related to pain sensation. These findings indicate that sensory nerves from the mid-cervical cord and sympathetic nerves from the cervical ganglion are distributed to the shoulder joint. It is possible that these nerves are related to symptoms such as pain in patients with "frozen" shoulder or other diseases.