The thin limbs of short and long loops of Henle of the rabbit kidney were studied by freeze fracture techniques. According to TEM studies of thin sections four segments are discernible: descending thin limbs of short loops, descending thin limbs of long loops, subdivided into an upper and a lower part, and ascending thin limbs (Kaissling and Kriz 1979). This division is supported by findings obtained with the freeze fracture technique and based on differences in the organization of the junctional complexes as well as on differences in the internal morphology of the cell membranes. The descending thin limbs of short loops have junctional complexes established by several closely arranged junctional strands and numerous desmosomes. The upper parts of the long descending thin limbs have tight junctions consisting of a variable number of strands; their outstanding characteristic after freeze fracture is a high density of intramembrane particles in both luminal and baso-lateral membranes. The tight junctions of the lower part of the long descending thin limbs consist of several anastomosing junctional strands, which are, in contrast, loosely arranged; the cell membranes contain only a sparse population of intramembrane particles. The ascending thin limbs are characterized by shallow tight junctions (frequently consisting of only one single junctional strand). Moreover, the epithelial cells of this segment are heavily interdigitated; thereby the tight junctions are correspondingly lengthened. In addition, this study presents further evidence that remarkable species differences occur among thin limb epithelia. The junctional complexes of the long descending thin limbs of the rabbit are organized quite differently from those of small rodents (e.g., rat, Psammomys). The data of this study support the concept that the tight junctions are the main determinant of ionic conductances of the paracellular pathway. However, with reference to recent findings from microperfusion studies, it becomes obvious that a correlation of the junctional morphology with the transepithelial water permeability is lacking, at least for the thin limbs.