Radiation transfer in the 6.3 μm H2O band in the mesosphere and lower thermosphere has been considered. For the day and night and for a set of vertical [H2O] profiles, the populations of the H2O vibrational states have been estimated. The populations have been used to estimate the zenith and limb total band radiance intensities for the 2.7 and 6.3 μm H2O bands in the altitude range 40-110 km. The calculated radiance intensities have been used for the analysis of rocket and satellite measurements of the emissions. © 1985.