A realistic parametrization scheme for the deposition of gravity-wave momentum in the middle atmosphere has been incorporated into the 2D version of a global-scale Numerical Spectral Model of the Earth's middle atmosphere. Here we present early results, obtained with only the simplest assumptions for the incident gravity-wave spectrum-that it is azimuthally isotropic (i.e., identical flux in the four cardinal directions), globally uniform, and unchanging with season-and with essentially ''untuned'' values of tunable parameters. This model reproduces reasonably well the observed anomalous latitudinal temperature distribution and the zonal circulation of the upper mesosphere during solstice, just as other models do. It also produces relatively large oscillations in the mean zonal circulation of the middle atmosphere at low latitudes, descending in altitude with time. In the mesosphere and upper stratosphere, the dominant period is semi-annual and the maximum amplitude is about 20 m/s near a height of 50 km. At lower levels, the dominant period is about 20 months and the maximum amplitude is about 8 m/s near 25 km. These values resemble those associated with the observed Semi-Annual Oscillation and Quasi-Biennial Oscillation, respectively, leading us to conclude that small scale gravity waves may contribute significantly to both. ¿ American Geophysical Union 1995