Electron microscopic and morphometric evaluation of the juxtaglomerular apparatus (JGA) of the human kidney showed the following: (1) The JGA of the human kidney consists of the epithelioid cells of afferent and efferent arterioles, the Goormaghtigh cells, and the macula densa, and it is abundantly supplied with sympathetic nerves. (2) The macula densa is in contact with the juxtaglomerular cell complex (JGC), the endocrine part of the JGA, via the Goormaghtigh cell field. The surface area of the macula densa is much larger than is the contact area with the JGA, totalling, in the healthy human kidney, some 66 cm2. (3) The total volume of all JGC's is 26 to 40 mm3. (4) Morphometric investigations of the JGC showed that appropriate stimulation by a decrease in the intrarenal or systemic blood pressure led to hypertrophy and hyperplasia of the JGC, which, in extreme cases, may also lead to a transformation of both the Goormaghtigh cells and mesangium cells into epithelioid cells.