The sinus node of the canine heart contains numerous autonomic nerve fibers. All of these are unmyelinated fibers associated with the cytoplasm of Schwann cell. The axon terminals, however, are devoid of such processes and are separated from the myocardial cells by an unusually wide space. In these naked nerve ending, 3 types of vesicles are observed: small granular vesicles, large granular vesicles, and agranular vesicles. The distribution of the vesicles by KMnO(4)-fixation has enabled us to classify autonomic nerve endings into 3 categories.