We have used computer simulation techniques to study the defect and diffusion processes in the perovskites SrTiO3, CaTiO3, and MgSiO3. The prediction of oxygen diffusion activation enthalpies in SrTiO3 agrees well with experimentally determined values thus justifying the use of such methods for materials with the perovskite structure. Calculations on defects and diffusion in MgSiO3 have been carried out at both zero and simulated high pressure in order to estimate the activation volumes for defect formation and diffusion. Our simulations predict that at pressures above 60 GPa, the nonstoichiometric MgO Schottky defect will be the most favorable and that Mg will replace Si as the slowest moving ionic species. The activation volume for Si diffusion is found to be negative. The magnitude of activation enthalpies for intrinsic defect processes is large, suggesting that extrinsic processes may well dominate defect formation and diffusion in MgSiO3 perovskite at depth in the Earth's mantle. ¿ American Geophysical Union 1993