High pressure and high temperature in situ X-ray observations of MgSiO3 perovskite were carried out using a newly developed Drickamer-type apparatus combined with synchrotron radiation. Variations of the unit cell parameters of the perovskite were observed as a function of temperature up to about 1900 K at 36 GPa. The orthorhombic distortion changed very little and no indication of transformations to higher symmetry phases was observed up to 1900 K. This result indicates that the orthorhombic perovskite is the stable phase of MgSiO3 in the most part of the lower mantle. The thermal expansivity of the perovskite is determined to be larger than 1.7(2)¿10-5 /K at 36 GPa and room temperature. This value will give an important constraint to the chemical composition of the lower mantle. ¿ American Geophysical Union 1993