A two-dimensional radiative/dynamical/chemical model is used to investigate the role of varying vertical diffusivity on the distribution and temporal variability of water vapor in the mesosphere. Model runs in which the effective turbulent Prandtl number varies over 2 orders of magnitude (values of 1, 10, and 100) are compared. The results indicate that a Prandtl number of 10 or more gives a simulation of mesospheric water vapor characterized by a strong decrease with height and a seasonal cycle with maximum mixing ratios during late summer. The test with a small Prandtl number gives, in contrast, very weak vertical gradients of concentration and a semiannual cycle with maxima in late winter and late summer. The very high Prandtl number case was included to test the role of vertical diffusion in the model when the effective diffusivity coefficient is very small. Comparison of these three cases indicates that the vertical structure of water is sensitive to the effective diffusivity over the range tested. The results also indicate that meridional advection by the mean circulation has a significant influence on the vertical distribution of water in the mesosphere. ¿1991 American Geophysical Union