The influence of ionization and conduction on the dispersion of acoustic gravity waves in the upper atmosphere has been investigated by means of an analysis of the corresponding dispersion relation, when the geomagnetic field is directed horizontally. It is shown that we have to distinguish two situations, i.e., acoustic gravity waves propagating perpendicular or nearly perpendicular to the Earth's magnetic field and acoustic gravity waves propagating oblique or parallel to the Earth's magnetic field. This last situation is the one considered in a paper by Hines and Hooke (1970). In the present paper both situations are described analytically; the transition between the two is studied numerically. A consequence of the incorporation of ionospheric conductivity is a damping of acoustic gravity waves with a damping rate lying between a characteristic damping rate that scales with the Pedersen conductivity and zero, depending on the frequency of the wave and the direction of propagation. This angular and frequency dependence of the damping rate may be experimentally verifiable. ¿ American Geophysical Union 1989