The role played by time-dependent mantle convection on exciting long-term polar motions is examined by means of a viscous model. For sufficiently low effective viscosity, polar-wander speeds of 0(1 deg/Myr) would require contributions from the off-diagonal elements of the inertia tensor, with magnitudes around 105 times smaller than those found in viscoelastic models used for postglacial rebound. Contributions from the large-scale mantle flow to the angular momentum vector of inertia tensor for non-linear rheology or for young planets. The relative roles of the two contributions depend on the non-linear rheology of the mantle and its convective vigor. ¿American Geophysical Union 1992