The nephron and collecting ducts of the little skate (Raja erinacea) and spiny dogfish shark (Squalus acanthias) have been investigated by light microscopy of semi-thin sections. Parts of the tubules (collecting ducts and distal segments) were identified after tubular injections with Microfil or carbon. The bundle zone was studied in serial sections. In the sinus zone transitions between the different segments were recorded. Thus, a complete reconstruction of the nephron, its subdivision into segments, and their localization in the kidney was accomplished. The nephron makes 4 loops. Beginning at Bowman's capsule, which sits between the bundle zone and sinus zone, the first loop is in the bundle zone. The nephron then extends into the sinus zone and turns back forming the second loop. This is followed by a third loop in the bundle zone which descends again into the sinus zone to form the last loop. The tail of the last loop (distal tubule) goes into the bundle zone and joins the collecting ducts. These collecting ducts are in the subcapsular connective tissue and progressively fuse to form a collecting tube. In the skate this tube traverses the thickness of the kidney between adjacent renal lobes to exit on the ventral kidney surface. In the shark, the large collecting ducts run on the surface of each lobe toward the medial margin of the kidney. Loops one and three and the early distal segment--all belonging to the same nephron--and a network of anastomosing capillaries form a bundle enclosed by a sheath of overlapping squamous cells termed "peritubular sheath." This anatomical unit forms the renal countercurrent system of the marine elasmobranch. The tubular bundle has a straight portion in which the nephron segments are arranged in a highly parallel fashion. The remainder of the bundle and of the encasing peritubular sheath are convoluted. The sequence of the tubule morphology beginning at Bowman's capsule is: neck segment (early and late), proximal tubule (four portions), intermediate (six portions), distal tubule (early and late), collecting duct (early and late).