Conclusions regarding the internal constitution of the terrestrial planets are dependent on the assumption as to the nature of the earth's core. It has previously been supposed that if the terrestrial planets, Earth, Venus, and Mars, are of similar composition the material of the core must represent a phase change, but if the core material is chemically distinct the planets must differ in over-all chemical composition. An equation of state for the mantle and core based on recent free oscillation and shock wave data is used in developing models of the terrestrial planets. It is demonstrated that Earth, Venus, and Mars can be satisfied with the hypothesis of chemical uniformity and a chemically distinct iron-rich core, provided that the external radius of Mars is about 3310 km. The radius of Mars could be as large as 3325 km and could differ only slightly from the gross composition of the earth, i.e. 2% less iron. Astronomical data indicate that Mars must be an almost homogeneous body, but compositional identity with the earth can be maintained by mixing mantle and core material.