Using semi-thin sections of marked tubules of the rat kidney, the distal part of the nephron is represented in toto for the first time and reconstructed in its course from the thick part of HENLE's loop to its junction with another collecting tubule. The shape of the model accords well with observations in vivo. On the basis of observations with light and electron microscopy, four types of cells are differentiated: Type 1 (Mittelstück) cell; type 2 (intermediate) cell; type 3 (light) cell and type 4 (intercalated) cell. According to the distribution of these different types of cells, the distal nephron is divided in the following way: Pars recta and pars convoluta of the distal nephron consist of type 1 (Mittelstück) cells; pars arcuata and pars recta of the cortical collecting duct exhibit type 3 (light) and type 4 (intercalated) cells; in the connecting portion of the distal tubule all four types are represented, type 2 (intermediate) and type 4 (intercalated) being the most frequent. The demarcation of the connecting portion according to our definition points to a dynamic relationship of the cell structures of the distal nephron, which gains clarity when conditions are experimentally varied. Hence the question for a morphological proof of where exactly the two nephrogenic blastemata join gives place to the more important question of what these cell structures and their variability mean as signs of functional adaptation-reactions.--The existence of the connecting portion, contested until now, is affirmed; a cytological characterization of it is given and a definition is recommended.