The distal segments beyond the macula densa--distal convoluted tubule, connecting tubule, cortical collecting duct--display cellular heterogeneity. The four different cell types, namely the DCT cell, CNT cell, the principal cell and intercalated cell differ mainly by the pattern of membrane amplification and they reveal also qualitative differences as to some cytoplasmic proteins. Each of the four cell types adapts to chronic changes in electrolyte metabolism with structural alteration, concerning essentially the membrane area over which the active transport step of the cell proceeds, in DCT-, CNT- and P-cells the basolateral cell membrane with the Na-K-ATPase, in intercalated cells the luminal cell membrane with a H+ ATPase. Since each cell type responds only to specific conditions with changes in membrane area and associated transcellular transport activity, morphological studies can help to determine the specific role of each cell type in the regulation of renal electrolyte excretion. Such investigations demonstrated that besides mineralocorticoid hormones the transport capacity of certain cells should depend on the solute composition of tubular fluid. Thus, changes in the transport pattern specifically induced in only one segment alters also the transport patterns of segments downstream. Cellular heterogeneity seems to guarantee the optimal regulation of renal electrolyte excretion.