The effects of air pollution on vegetation may provide an important control on the carbon cycle that has not yet been widely considered. Prolonged exposure to high levels of ozone, in particular, has been observed to inhibit photosynthesis by direct cellu lar damage within the leaves and through changes in stomatal conductance. We have incorporated empirical equations derived for trees (hardwoods and pines) and crops into the Terrestrial Ecosystem Model version 4.3 (TEM 4.3) to explore the effects of ozon e on net primary production and carbon sequestration across the conterminous United States. Our results show up to a 5% reduction in Net Primary Production (NPP) in response to modeled historical ozone levels during the late 1980s to early 1990s. The lar ge st decreases (over 20% in some locations) occur in the eastern U.S. and Midwest, during months with high ozone levels and high productivity. Carbon sequestration during the 1980s is reduced by 30 to 70 Tg C/yr with the presence of ozone, or 5 to 23% o f recent estimates of the total carbon sequestration for the U.S. Thus the effects of ozone on NPP and carbon sequestration should be factored into future calculations of the U.S. carbon budget.Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change Website. (http://mit.edu/globalchange/www/)Includes bibliographical references (p. 20-21)