Earth, Mars, and Venus formed at about the same time and in the same region of the solar nebula. It is therefore reasonable to assume that their initial composition and tectonic activity were similar. I use this as the major assumption in my study of the early atmospheres of these planets. A primary goal was to estimate the partial pressure of CO(,2) in the early atmospheres. This problem was approached in an inverse sense. I used a one dimensional radiative-convective model of the atmosphere to determine what levels of CO(,2) would be needed to maintain the climatic conditions that are thought to have existed on these planets 4.0 billion years ago. The range of possible atmospheric compositions is large for any one planet, but narrows considerably when the composition must be compatible with conditions on all three planets. From a comparative analysis of the planets, I estimate that 4.0 billion years ago the surface partial pressure of CO(,2) on Mars was between 1.3 and 2.1 bars and its partial pressure on Earth may have been as high as 14 bars. The Earth and Martian atmospheres were very stable and would not have gone into a runaway greenhouse state even if the CO(,2) partial pressure had been equivalent to 100 bars on Earth. On the other hand, because of its proximity to the sun, radiative-convective equilibrium could not be reached on early Venus for CO(,2) partial pressures less than about 3.5 bars. For surface pressures up to 11 bars, the Venus atmosphere was very susceptible to the rapid photodissociation of water vapor and the subsequent escape of hydrogen. At partial pressures greater than 15 bars of CO(,2), the increased albedo of Venus due to Rayleigh scattering dominates and the atmosphere becomes stable against a runaway greenhouse state. This stabilizing effect of Rayleigh scattering in massive CO(,2) atmospheres, along with the relative distance of the Earth and Venus from the sun played an important role in the very divergent evolutions of the terrestrial atmospheres.