The structure and innervation of the juxtaglomerular apparatus are examined by both light and electron microscopy, using series of semithin and thin sections. At the vascular pole of the renal corpuscle, granulated and nongranulated epithelioid cells, Goormaghtigh's cells and mesangial cells, as well as adjacent smooth muscle cells, are all connected to each other. Due to their fine structure, their topographical conti- nuity, and the numerous gradual stages of transformation, the different components of the vascular wall are defined as ramified smooth muscle cells (epithelioid cells), which are separated from typical smooth muscle cells of glomerular arterioles. The modified smooth muscle cells show not only numerous cytoplasmic processes, but also many plasmalemmal infoldings. In some areas, these exhibit a spiny differentiation similar to that of coated vesicles. Particles of condensed material resembling basal lamina are often found in the tubular and lamellar indentations of granulated epithe- lioid cells. The mechanism of release of secretory granules and the mode of secretion via coated vesicles is discussed. Using exogeneous protein horseradish peroxidase to mark the extracellular space, gap junctions between all types of cells of the vascular wall may be seen. The func- tional significance of an electrotonic ally coupled cell group and the role of the epithe- lioid cells as stretch receptors and pace-maker elements, respectively, is discussed in connection with the autoregulation of the nephron.