Using immunohistochemistry, we studied the origins and pathways of parasympathetic and sensory nerve fibers to the pial arteries in four squirrel monkeys. Following its application to the surface of the middle cerebral artery, the retrograde axonal tracer True Blue accumulated in parasympathetic neurons of the sphenopalatine ganglion and the internal carotid ganglion. The latter is strategically located where the internal carotid artery enters the cranium. Fibers from the sphenopalatine ganglion reach the internal carotid artery in the cavernous sinus region after running as rami orbitales. Before reaching the internal carotid artery, the fibers bypass aberrant sphenopalatine ganglia, with the most distant, the cavernous ganglion, being located in the cavernous sinus region. True Blue also accumulated in sensory neurons of the ophthalmic and maxillary divisions of the trigeminal ganglion and in sensory neurons of the internal carotid ganglion. Fibers from the ophthalmic division of the trigeminal ganglion reach the internal carotid artery as a branch through the cavernous sinus, bypassing the cavernous ganglion. Fibers from the maxillary division also bypass the cavernous ganglion after reaching it via a recurrent branch of the orbitociliary nerve. Thus, the cavernous ganglion forms a confluence zone for parasympathetic and sensory fibers in the region. In addition, parasympathetic and sensory fibers leave the confluence zone to follow the abducent and trochlear nerves backward to the basilar artery and tentorium cerebelli, respectively. Clinical implications are discussed. (Stroke 1991^2:331-342) B lood vessels of the brain surface, and to some extent those of its parenchymal branches, are supplied with sympathetic, parasympathetic, and sensory nerve fibers, spreading like a network in the adventitia.1 It has been long known that all sympathetic fibers (except those in the caudal basilar artery in some species) originate in the superior cervical ganglion.1 An origin for parasympathetic fibers in the sphenopalatine and otic ganglia has been demonstrated in rats and cats by the retrograde axonal tracer technique, by nerve section, and by histochemical mapping of the putative transmitters acetylcholine (cholinesterase, choline acetyltransferase [ChAT]) and vasoactive intestinal polypeptide (VIP).2 -5 By this latter technique (histochemical staining of the putative transmitters substance P and calcitonin geneFrom the