Since the anatomical descriptions of Bowman showing differences between nephrons originating in the superficial and deep cortex, the concept of heterogeneity has been extended from identification of dissimilarities between nephrons to recognition of inhomogeneity within major portions of individual nephrons. We are now aware of functional correlates for the anatomical differences between nephrons, between analogous parts of different nephrons, and between the three portions of the proximal tubule and the three or more parts of the distal tubule. The implications of all of these differences for major renal processes, such as isosmotic fluid transport, salt balance, hypertension, urinary acidification, and the concentration or urine are now being defined. It seems likely that new conceptual and technical approaches, especially electron probe microanalysis, will add appreciably to defining the role of heterogeneity in these and other processes. Despite the increasing complexity of nephron heterogeneity, it is recommended that our basic nomenclature be retained and that new findings be incorporated into the schema set forth by Karl Peter. It would be very helpful if reports of investigations on single nephrons or segments of nephrons were to include diagrams delineating the structures on which the work was performed.