The spectral mesosphere/lower thermosphere model (SMLTM) is an extension into the thermosphere (up to ~220 km) of the three-dimensional middle atmospheric (~15--120 km) model developed by Akmaev et al. [1992>. The model incorporates realistic parameterizations of radiative transfer and dissipation processes including eddy and molecular viscosity and thermal conductivity and ion drag. A discrete multicomponent gravity wave parameterization is optimized with respect to the zonal mean temperature distribution as represented in the CIRA-1986 empirical model. The diurnal tide is generated by forcing specified at the model lower boundary and by in situ absorption of solar radiation. The semi-diurnal tide is primarily generated by ozone absorption of solar UV radiation in the stratosphere. First results of tidal simulations indicate that even without any additional tuning the simulations compare well with recent observations from the Upper Atmosphere Research Satellite (UARS). This means that the background temperature and wind distributions, gravity wave momentum deposition, turbulent diffusion, and tidal forcing all interact self-consistently and are in agreement with observational data. It is noted that simultaneous UARS wind and temperature measurements, if they become available, will provide key information needed to distinguish the primary mechanism for damping the diurnal tide. ¿ American Geophysical Union 1996