Theoretical calculations of diffusion in periclase were performed to model the rheological properties of the lower mantle. Gibbs free energy values derived from molecular dynamics simulations using a non-empirical interatomic potential to accurately predict diffusion in MgO at zero pressure. Diffusion coefficients were computed for pressures and temperatures up to 140 GPa and 5000 K. We find that Mg vacancies required for diffusive transport are likely extrinsic (due to impurities), and the resulting O vacancies would then be too few to support bulk O transport. Estimates of viscosity from these results are consistent with those inferred for the lower mantle from geophysical observations if grain boundary diffusion is responsible for O transport. ¿ 1998 American Geophysical Union