The juxtaglomerular apparatus, located in the glomerular hilum, consists of a vascular component (afferent and efferent arterioles and extraglomerular mesangium) and a tubular component (macula densa). Two types of contact between vascular and tubular components are observed: a) a complex type, involving distal tubule, extraglomerular mesangium, and proximal efferent arteriole, and b) a simple type, consisting of apposition of the basement membranes of the vascular and tubular components. Juxtaglomerular granular cells, the source of renin, are present throughout the vascular component but are more numerous in the afferent arteriole. They can be considered as "myoendocrine" cells, since they contain myofibrils and attachment bodies, together with secretory granules and crystalline protogranules. Macula densa cells differ from those elsewhere in the distal tubule in that their nuclei are closer to each other, the Golgi apparatus is basally located, and their basal membrane infoldings are less prominent. Adrenergic nerves are demonstrable by fluorescence histochemistry in the juxtaglomerular region. Electron microscopy reveals unmyelinated nerve fibers containing small dense-cored vesicles and capable, as shown by ultrastructural autoradiography, of incorporating exogenous tritiated norepinephrine. Neuroeffector junctions occur between nerves and cells of the vascular and, less frequently, the tubular component. In addition, adrenergic axons are observed in a juxtaglomerular cell tumor. Nerve terminals are seen in direct contact with the tumor cells.