Because of the availability of knockout mouse models to examine renal transport mechanisms, it has become increasingly important to describe the cellular distribution of major renal transporters in mice. We have used immunocytochemistry and freeze-fracture electron microscopy to compare the renal distribution of aquaporin-4 (AQP4) with that previously described in rat. In rat kidney AQP4 is present exclusively in basolateral membranes of collecting duct principal cells. In mice, however, AQP4 was also detected by immunocytochemistry in basolateral membranes of proximal tubule S3 segments, and not detected in S1 and S2 segments of proximal tubule. Freeze-fracture electron microscopy revealed orthogonal arrays of intramembrane particles (OAPs) on the basolateral membranes of the S3 segment. In AQP4-knockout mice, immunostaining was absent and OAPs were found neither in collecting ducts nor in the S3 segment of the proximal tubule. The urinary concentrating capacity after deletion of both AQP1 and AQP4 was further reduced compared with that of AQP1 or AQP4 null mice, suggesting an additive effect of AQP1 and AQP4 in the concentrating mechanism. The functional significance of the apparent species-dependent expression of AQP4 in proximal tubules is unknown, but may relate to physiological differences between rats and mice.