The qualitative and quantitative morphologic features of the cells lining the inner medullary collecting duct (IMCD) in the outer (IMCD1), middle (IMCD2) and inner (IMCD3) segments were investigated. Kidneys of male rats were fixed by in vivo vascular perfusion with glutaraldehyde and processed for light microscopy and scanning and transmission electron microscopy. The IMCD1 consisted of both principal cells and intercalated cells similar to those present in the outer medullary collecting duct. The principal cells were covered with small microvilli and a single cilium. Most of the IMCD2 and the entire IMCD3 contained one cell type (IMCD cell). When compared with the principal cells, the IMCD cells were taller, had fewer basal infoldings and a lighter staining cytoplasm containing numerous free ribosomes and small electron-dense cytoplasmic bodies in the basal region. The luminal surface was covered with prominent microvilli, but had no cilia. Morphometric analysis demonstrated that the surface density of apical and basal plasma membranes decreased from IMCD1 to IMCD3. However, because of an overall increase in tubule volume from IMCD1 to IMCD3, there were no significant differences in the absolute area of apical or basal membranes between the three segments. In contrast, the absolute area of lateral membranes increased significantly from IMCD1 to IMCD3. This study demonstrates that the IMCD1 consists of principal cells and intercalated cells similar to those in the outer medullary collecting duct, whereas the cells in most of the IMCD2 and the entire IMCD3 appear to represent a distinct and separate cell type which we choose to call the IMCD cell. Thus, both morphologic and functional heterogeneity appear to exist along the IMCD.