A transgenic mouse approach was used to examine the mechanism of principal cell-specific expression of aquaporin-2 (AQP2) within the renal collecting duct. RT-PCR and immunocytochemistry revealed that murine AQP2 was expressed in principal cells in the renal collecting duct, epithelial cells of the vas deferens, and seminiferous tubules within testis. The vas deferens expression was confirmed in rats. RT-PCR and immunocytochemistry showed that 14 kb of the human 5′-flanking region confers specific expression of a nucleus-targeted and epitope-tagged Cre recombinase in the principal cells within the renal collecting duct, in the epithelial cells of the vas deferens, and within the testis of transgenic mice. These results suggest that cell-specific expression of AQP2 is mediated at the transcriptional level and that 14 kb of the human AQP2 5′-flanking region containcis elements that are sufficient for cell-specific expression of AQP2. Finally, renal principal cell expression of Cre recombinase is the first step in achieving cell-specific gene knockouts, thereby allowing focused examination of gene function in this cell type.