Small-scale gravity waves (GWs) originating in the lower atmosphere maintain the large-scale structure and circulation in the mesosphere and lower thermosphere (MLT). At solstice the drag imposed by these waves supports a strong meridional circulation from summer pole to winter pole that acts as a giant refrigerator transporting heat from a colder summer mesosphere to a warmer winter mesosphere. The Doppler-spread parameterization (DSP) of gravity waves developed by C. O. Hines has been implemented into the Spectral Mesosphere/Lower Thermosphere Model. Simulations for solstice and equinox conditions compare favorably with empirical models and observations. A particularly good agreement for both seasons is found with recent in situ observations of polar mesospheric temperatures. The thermal structure at high latitudes, especially at solstice, is almost entirely driven by the GW momentum deposition in the MLT. The DSP appears to provide a very realistic distribution of the GW drag imposed on the large-scale circulation. A modification is suggested for the calculation of GW energy deposition rate that appears to be more consistent with the physics of Doppler spreading. The simulations exhibit strong seasonal variations in the energy deposition rates with a summer maximum of ~200 mW kg-1 at middle and high latitudes in the upper mesosphere in very good agreement with recent in situ estimates. ¿ 2001 American Geophysical Union