Water is a critical component of all living cells. Interestingly, tissue membranes show a great degree of water permeability. Mammalian red cells, renal proximal tubules, and descending thin limb of Henle are extraordinarily permeable to water. Water crosses hydrophobic plasma membranes either by simple diffusion or through a facilitative transport mechanism mediated by special protein “aquaporin”. Over the last decade, genes for several members of aquaporin family have been cloned, expressed, and their distribution studied in many tissues. A new member of AQP family, Aquaporin-9 (AQP9), has been cloned from human leukocytes by homology cloning. It is mostly homologous with AQP3 and AQP7 (54–48% identity). AQP9 (295 AA) is primarily expressed in peripheral leukocytes, and lesser amounts in liver, lung and spleen. AQP9 is permeable to water and urea. Unlike AQP3 and AQP7, it did not facilitate glycerol transport. AQP families of proteins are predicted to contain six transmembrane domains. The N and C-terminus are predicted to be cytoplasmic.