Background: Bladder urothelium, considered a poorly permeable urine-blood-barrier, expresses transmembrane channels called aquaporins (AQPs). AQPs are important for transepithelial water and solute movement in various tissues and their discovery in bladder urothelium supports a more controversial hypothesis that urothelium is able to modify the composition and volume of urine in the lower urinary tract. Thus, AQPs may act as potential targets for treatment of debilitating conditions such as nocturia. The aim was to investigate the expression of AQPs in porcine bladder, a valuable alternative to human bladder for elucidating physiological principles. Methods: 6-month old female pig bladders were obtained from the local abattoir. PCR was carried out on the c-DNA synthesized from total RNA isolated from the mucosa, and also its component urothelium. Primers were designed for the Sus scrofa AQPs 1-11. PCR products were separated by electrophoresis and sequenced. For immunohistochemical studies, sections of fresh intact bladder dome were fixed (4% neutral buffered formalin), processed and embedded in paraffin. Tissues were sectioned at 3 µm and placed on charged slides. After blocking endogenous peroxidase activity and subsequent antigen retrieval, tissue sections were incubated with primary antibodies (dilutions made up in PBS+0.5% Triton and 1% horse serum) overnight at 4°C. Detection used an avidin biotin peroxidase system and a diaminobenzidine tetrahydrochloride chromogen. Negative controls replaced the primary antibody with the relevant peptide or tris-buffered saline. Sections were counterstained with haematoxylin and examined using a Nikon Eclipse 50i microscope. Results: AQP 1, 3, 9 and 11 mRNA expression were found in the mucosa and in the urothelium itself. AQPs 2, 5, 6, 7, 8 and 10 expression were not detected at the mRNA level. Immunohisctochemical analysis demonstrated the expression of AQP 1 in the endothelial layer of arterioles in the sub-urothelial layer. AQPs 3 and 11 were expressed throughout the urothelium and AQP 9 immunoreactivity was only detected in the umbrella cells of the urothelium. Conclusion: Four different subtypes of the AQPs (1, 3, 9, and 11) have been identified in pig bladder, suggesting that AQPs may play an important role in regulating urothelial cell volume and osmolality and determining the final composition of urine. Changes to the extracellular osmolality of urothelial cells may act as a form of stress which could result in release of various transmitters, affecting afferent nerves in the mucosa, as well as modulating spontaneous contractile activity of the bladder wall. However, the exact role of AQPs in mediating the sensory and contractile functions of the bladder wall, and the mechanisms by which their expression and function in the urothelium is regulated remains to be elucidated.