The equilibrium distribution function of water in the aquaporin-Z is calculated based on the three-dimensional reference interaction site model theory. It is found that water in aquaporin-Z distributes continuously throughout the channel with the open structure, while the distribution has a conspicuous gap around the R189 side chain in the closed form. The gating mechanism of the channel is inferred from the water distribution. The method also provides information with respect to the distribution of hydrogen atoms of water inside the channel, which is important to investigate the conduction mechanism of molecules including protons in the channel. (c) 2007 Elsevier B.V. All rights reserved.