The fine structure of the juxtaglomerular apparatus in the toad, Bufo bufo was investigated.—It is suggested that the granules in the media cells of the afferent arteriole are formed from the Golgi apparatus. Many granules have a content of lamellar material. The media cells do not show the ultrastructural features of active secretory cells. In the media cells, the myofilaments are situated near the vascular lumen. The other cell organelles, including the granules, are preferentially located at the opposite pole of the cell in the neighbourhood of the macula densa cells and the adventitial nerve fibres. In these regions the media cells show many pinocytotic vesicles. The nerve fibres innervating the juxtaglomerular cells are non-myelinated and their varicosities contain dense core vesicles. The basement membranes of media cells and adjacent macula densa cells occasionally fuse, which may indicate a functional relationship between these cells.