Byline: Maria De Fatima Andrade, Rita Y. Ynoue, Robert Harley, Antonio H. Miguel The main sources of ozone precursors in the atmosphere of the Sao Paulo Metropolitan Area (SPMA) in Brazil are emissions from gasoline-, ethanol-, and diesel-powered vehicles. Ozone is a significant air quality problem in the SPMA. To come into compliance with the National Ambient Air quality Standard for Ozone, emission reduction policies must be established. An adequate emissions inventory and a description of the meteorology and chemistry involved are needed in order to evaluate the effectiveness of methodologies that reduce the effect of vehicle emissions on ozone formation. During the period 1012 August 1999, concentrations of ozone in the SPMA and the impact of the official emissions inventory were simulated through application of an urban-scale Eulerian model. We found that using the official inventory in simulations resulted in ozone values much lower than those observed and nitrogen oxide emission profiles that were overestimated by a factor of approximately two.