A comprehensive three-dimensional mesoscale model, with detailed treatment of meteorology and photochemistry, is applied to the eastern United States and southeastern Canada to address the issue of ozone production over rural regions. Simulations with various anthropogenic emission reductions over a 4-day high-pressure period, for which the eastern U.S. experienced elevated O3 levels, are compared to the results of a reference case that incorporates the most recent National Acid Precipitation Assessment Program (1985) emissions inventory. Our focus is on the relative effects of anthropogenic NOx versus nonmethane hydrocarbons (NMHC) emission reductions on O3 concentrations within this photochemical regime. The model results are consistent with earlier studies that predict a reasonably high O3 sensitivity to NMHC emission reductions, and O3 increases for NOx emission reductions in regions of strong urban influence. However, the total area that exhibits these distinctly urban characteristics comprises less than 10% of the continental area within the model. O3 production for the majority of the eastern United States is found to be limited by NOx availability rather than NMHC. Thus the sensitivity to NOx emission reductions averaged over the continental regions of the model domain is about a factor of 3 higher than that of NMHC. Because of large uncertainties in the natural and anthropogenic NMHC emission, additional sensitivity runs are performed with increased anthropogenic NMHC emissions and natural NMHC emissions excluded. These alterations do not effect our basic conclusions with respect to rural O3 formation but do impact urban locations. When anthropogenic NMHC emissions are increased by a factor of 4, control of NOx emissions has nearly the same effect in both urban and rural areas. The nonlinear nature of regionally averaged O3 with respect to anthropogenic and natural precursors is also illustrated from the results of the sensitivity studies. ¿ American Geophysical Union 1991