Characteristics of the structural organization of the afferent and efferent glomerular arterioles of the mammalian kidney, that have so far escaped adequate functional discussion, include: presence and distribution of myoendothelial junctions in afferent and efferent arterioles and their possible functional relevance as stretch receptors; distribution of smooth-muscle cells around cortical efferent arterioles and, in particular, the presence of a contractile segment at the beginning of the efferent arterioles; elaboration of a basement membrane-like material beneath the endothelium of efferent arterioles and the continuity of this material with the mesangial and extramesangial matrix material; and structural organization of the endothelium of juxtamedullary efferent arterioles, consisting of up to 30 longitudinally arranged endothelial cells, and its possible functional significance.