The elastic constants and equation of state of MgSiO3 perovskite and MgO are calculated as functions of pressure and temperature using the potential induced breathing (PIB) model, which is an ab initio model in the sense that no experimental data are used. Excellent agreement with experiment is found for the equation of state of perovskite. The calculated thermal expansivity is lower than determined experimentally. PIB provides pressure derivatives of elastic constants that are probably more accurate than experimental results, so that PIB pressure derivatives may be used with experimental elastic moduli when these become available for MgSiO3 perovskite. A fourth order finite strain equation is required to fit the elastic moduli, since the shear modulus has only a small pressure dependence at high pressures. The calculated elastic properties are used to discuss the composition of the lower mantle. The results are consistent with a lower mantle composition enriched in Fe. ¿ American Geophysical Union 1987