The sympathetic innervation of these ducts was studied by means of the Falck fluorescence histochemical technique and electron microscopy. A loose network of sympathetic fibers was observed on the surface of the lymphatic duct, yet there were large areas of the duct wall void of any fibers. The majority of the fibers branched from the plexuses of sympathetic fibers surrounding the vasa vasorum of the ducts. Sympathectomy experiments demonstrated the fibers do not travel up the vagosympathetic trunk or the carotid artery to reach the ducts, but apparently are carried along the plexuses of sympathetic fibers surrounding the smaller blood vessels outside the carotid sheath. The individual branches not accompanying any blood vessels would travel along the duct for distances up to several hundred microns, with occasional branches, to terminate without any specializations. The nerve fibers were encased in a Schwann cell sheath which in some cases only partially surrounded the fibers. Several of the axon profiles contained synaptic vesicles including small granular (or dense-cored) and agranular vesicles and a few large synaptic vesicles. In all cases observed, the axons and their Schwann cell sheath were located in the adventitia outside the smooth muscle cells.