The three-dimensional ultrastructure of principal cells in the rat renal collecting duct was studied by scanning electron microscopy (SEM) using the NaOH digestion technique and the aldehyde prefix-osmium-DMSO-osmium method, as well as by transmission electron microscopy (TEM). Special reference was given to the basal labyrinth of the cells and its numerous ruffles and infoldings of the basal plasma membrane. Observations showed that, as the collecting duct descends from the cortical collecting duct (CCD) to the terminal portion of the inner medullary collecting duct (t-IMCD), the pattern of the labyrinth gradually simplified and the ruffles grew thinner. In the CCD, the labyrinth was conspicuously complicated in structure, being formed of tightly arranged ruffles of uniform shape and thickness (70 nm). From the outer medullary collecting duct (OMCD) to the initial portion of inner medullary collecting duct (i-IMCD), the labyrinth became less complicated due to the mingling of wide flattened ruffles. Also, the basal infoldings were reduced in depth (from 700 nm in CCD to 500-600 nm in i-IMCD). In the t-IMCD, the labyrinth was rudimental, and instead presented small grooves (300 nm in depth) which corresponded to indentions of the basal plasma membrane. The regional simplification of the labyrinth was accompanied by morphological changes in mitochondria suggesting their functional decline: the electron density and number of cristae were reduced, these being changed in shape from plate-like to vesicular. These morphological data readily account for the potential for active transport by the collecting duct, which is highest in the CCD and is decreased towards the t-IMCD, and which may function merely as an excretory duct of urine from the papilla. The present study three-dimensionally demonstrates fine-structural heterogeneity in different segments of the collecting duct of the rat kidney.