The distribution of sympathetic fibers and calcitonin gene related peptide (CGRP)-positive fibers was examined in the endolymphatic sac of 10 guinea pigs by using immunohistochemical techniques to demonstrate tyrosine hydroxylase (TH) and CGRP. A meshwork of TH-positive fibers, found around the sigmoid sinus, sent branches to the distal and the intermediate parts of the endolymphatic sac. In these areas. TH-positive fibers traveled freely from blood vessels and formed a loose plexus beneath the lining epithelium. Such fibers were rare in proximal portion of the endolymphatic sac and in the endolymphatic duct. Seven days after elimination of the ipsilateral superior cervical ganglion, some of the TH-positive fibers were gone, however, there were still a few fibers in the endolymphatic sac as well as around the sigmoid sinus, thus their origin remains obscure. CGRP-positive fibers also branched from the fibers around the sigmoid sinus, and were distributed throughout the endolymphatic sac, some occasionally extending to the endolymphatic duct. They not only formed a dense plexus in the sublining space, but also spread through the lining cell layer. Elimination of the ipsilateral superior cervical ganglion did not affect the distribution of CGRP-positive fibers, indicating that they are probably not sympathetic fibers.