The renal collecting duct exhibits cellular heterogeneity. An ultrastructural analysis of this nephron segment during maturation has been performed to determine morphological characteristics of developing principal and intercalated cells (IC). Apical surface features and cytoplasmic ultrastructure were best revealed by scanning electron microscopy and by transmission electron microscopy, respectively. Morphological maturity was assessed by comparing measurements of apical perimeter, vesicular profiles, and mitochondrial volume percent. In mature cortical collecting duct (CCD), approximately 30% of total cells were rich in mitochondria and generally dark in matrix staining, i.e., IC. Most IC in outer cortex were microvillated, whereas in inner cortex most IC were microplicated with interspersed microvilli. Only approximately 16% of cells in outer medullary collecting duct (OMCD, outer stripe) were IC, with light cytoplasm and large protruding apical surfaces covered exclusively with microplicae. Neonatal CCDs showed no IC in outer cortex; a single type of principal-like cell was found in outer cortical region. Neonatal IC in both CCDs and OMCDs showed smaller apical perimeters, vesicular volumes, and mitochondrial volume percents than did mature IC. Whereas the number of cortical IC was reduced in the newborn, IC were present in mature numbers in OMCD and appeared relatively more mature than did IC of CCD. Some (approximately 10%) of IC in neonatal CCD did not show an identifiable apical surface pattern. Principal cells from both newborn CCD and OMCD were also morphometrically immature. It is clear that the neonatal collecting duct undergoes postnatal proliferation and differentiation.