Original work on the renal structures in three classes of lower vertebrates is presented. By the application of light and electronmicroscopy, histochemistry and the compilation of available information on renal physiology and morphology, further evidence for the homology of nephron segments is forwarded. The results have been correlated with the habitat and evolutionary kinship of the lower vertebrates. A general evolutionary line can be drawn from early vertebrates (e.g. Petromyzontia) via archaic fresh-water fish (e.g. Polyteridae) and lungfish to the amphibian nephron. Renal structures of Elasmobranchs are very complex, but can be compared with the more primitive nephron of the Polyteridae. The morphological basis for the unique ability of the marine Elasmobranchs to build up a high urea concentration in the blood is given by the description of the renal countercurrent system in the lateral bundle zone. The intercalated cells which are prominent in the distal nephron of the mammalian kidney have been traced back via the lungfish and the polypterids to the Petromyzontia. The teleost kidneys appear highly specialized and supply a host of animal models for the study of renal secretion. The morphological information with the compilation of literature on renal physiology in fish is relevant for the understanding of the evolution of the vertebrate kidney and serves as a presentation of animal models for the study of basic renal mechanisms.