A new model has been constructed for calculating the level of atmospheric CO(2) during the past 570 million years. A series of successive steady states for CO(2) is used in order to calculate CO(2) level from a feedback function for the weathering of silicate minerals. Processes considered are: sedimentary burial of organic matter and carbonates; continental weathering of silicates, carbonates, and organic matter; and volcanic and metamorphic degassing of CO(2). Sediment burial rates are calculated with the use of an isotope mass-balance model and carbon isotopic data on ancient seawater. Weathering rates are calculated from estimates of past changes in continental land area, mean elevation, and river runoff combined with estimates of the effects of the evolution of vascular land plants. Past degassing rates are estimated from changes in the rate of generation of sea floor and the shift of carbonate deposition from platforms to the deep sea. The model results indicate that CO(2) levels were high during the Mesozoic and early Paleozoic and low during the Permo-Carboniferous and late Cenozoic. These results correspond to independently deduced Phanerozoic paleoclimates and support the notion that the atmospheric CO(2) greenhouse mechanism is a major control on climate over very long time scales.