The formation of photooxidants and aerosols in the Milan area (Po Valley, Italy) has been simulated for 13 May 1998 with an Eulerian model. The high-pressure conditions and the weak synoptic circulation favor the formation of strongly enhanced photooxidants and aerosol levels downwind of the metropolitan area of Milan. The polluted air masses are transported northward where the highest ozone (O3) concentrations of up to 200 ppb are reached 30 to 40 km away from the city center of Milan. The core of the Milan plume has an extension of only 10 km in length and 15 km in width at 1600 LT. The focus of the simulation was the sensitivity of the O3 formation to the reduction of nitrogen oxides (NOx) and volatile organic compounds (VOC) emissions. The model results are compared to the measurements in order to evaluate the quality of the simulation. The spatial and temporal variations of the concentrations of O3 and other trace compounds are reasonably well reproduced. Several reasons for systematic deviations have been analyzed in order to understand their possible influence on the sensitivity analysis. Moreover, the simulation showed that indicator species such as the H2O2/HNO3 or O3/NOy ratio could be applied to characterize the VOC/NOx regime of the O3 production in the Milan area, but that it is necessary to modify the indicator ratios to include the aerosol nitrate. Highest O3 values appear at the transition from VOC to NOx sensitivity approximately 30 km downwind of the highest emissions.