Because of the conservation of angular momentum, the atmospheric winds and the mass exchange between the Martian ice caps and atmosphere, associated with the sublimation/condensation process (mainly CO2), induce seasonal effects on Mars' polar motion, nutation, and length of day (LOD). These effects are computed using the output of a global circulation model of the Martian atmosphere, providing atmospheric pressure fields, ice cap surface pressure fields, and zonal as well as meridional winds. For the LOD variations, total amplitudes (CO2 and wind effects) of 0.22 ms for the annual wave and of 0.38 ms for the semiannual wave are obtained. These amplitudes are more than one order of magnitude larger than the LOD variations induced by the zonal tides, which are at the level of 10 μs. For the induced polar motion the annual amplitude is ~11 milliarcseconds (mas), and the semiannual amplitude is ~3 mas. The effect on the nutations, related to the diurnal forcing, is at the level of 0.1 mas. The differences between the results for a liquid and for a solid core are examined and shown to be <1% of the total effects. ¿ 2000 American Geophysical Union