The movement of an ion through a membrane channel proceeds in at least five separate steps: Diffusion through the aqueous phases up to the channel, association with the channel itself, translocation through the channel, dissociation from the channel, and diffusion through the aqueous phases out from the channel. We demonstrate that, contrary to current working assumptions, the aqueous diffusion step may be an important determinant of overall ion movement through the channel. We further describe the kinetics of Na+ movement through gramicidin A channels. Using these data we show that one will have to consider the movement of H2O through the channel explicitly in any complete model for ion translocation through the channel interior.