The distribution of nerve fibers in the arterial wall of both the elastic-type artery and muscle-type artery was studied by rabbits with the light and electron microscope. A noteworthy finding with the elecron microscope was the connection of smooth muscle cells with the unmielynated nerve ending observed in the middle layer of the tunica media of the common carotid artery of the rabbit, and the cell membrane of the nerve ending and the cell
membrane of smooth muscle cells is separated by a space of about 500A and the space is filled with a substance of a relatively high electron density. At this nerve ending there can de observed two synaptic vesicles and a neurofilament within the axon. The electron density of such a vesicle is high. In the space between the cell membrane of the ending and the cell membrane of smooth muscle cell there are several synaptic vesicles released. The size of these synaptic vesicles is approximately the same as the largest vesicles among various synaptic vesicles observed in the tunica adventitia of the a. carotis interna of the same rabbit.