A three-dimensional model of the middle atmosphere is introduced. The model is based on the full set of the primitive equations. It is designed to simulate a yearly cycle of the middle atmosphere. Results are presented for the solstice and equinox conditions. The model reproduces the main observed features of the middle atmospheric circulation: the stratospheric-mesospheric jet streams and the cold summer mesopause region at solstice with reversed zonal wind especially in the summer mesosphere, and the weak westerly circulation at equinox. The parameterized effects of breaking gravity waves in the mesosphere drive the atmosphere out of radiative balance. They lead to a meridional circulation with a one-cell structure at solstice with upward (downward) motion over the summer (winter) pole and a meridional flow towards the winter hemisphere and a two-cell structure at equinox with upward motion over the tropics and downward motion over the polar regions. Potential fields are presented for horizontal vector fields. They suggest that the stratospheric circulation can is dominated by horizontally nondivergent flow. This is modified by the results of a more quantitative view at the interaction of planetary waves on the zonal mean flow, which clearly identifies the essential role of horizontal divergence on the stratospheric circulation. In the mesopause region the forces induced by the breaking gravity waves are of the same order as the horizontal pressure gradient force or the Coriolis force, so that geostrophic theory can no longer be applied here. This is confirmed by a comparison between idealized tracers and the dynamical tracers potential temperature and isentropic potential vorticity (IPV). The IPV is no longer a conservative tracer in the mesopause region. ¿ American Geophysical Union 1993