The application of modern techniques in molecular genetics to classic diseases in clinical nephrology is highlighted by the recent description of the molecular basis of Barrier's and Gitelman's syndromes, A series of detailed studies are described that have resulted in the identification of specific mutations in four different genes, each of which causes hypokalemic alkalosis, salt wasting and hypotension. The importance of these genetic studies in understanding renal physiology and the regulation of blood pressure, and in developing new therapeutic strategies is discussed.