Aircraft measurements of ozone and the oxides of nitrogen have characterized the horizontal and vertical extent of the urban plume downwind of Birmingham, Alabama. Derived NOx emission rate estimates of 0.6×1025 molecules s−1, with an uncertainty of a factor of 2, for this metropolitan area are in reasonable accord with the 1985 National Acid Precipitation Assessment Program inventory, which gives 1.2×1025 molecules s−1 for daytime emissions. These estimates are from two flights in 1992 when the urban plume was well separated from the plumes from two power plants northwest of the city. During three flights in 1990 the plumes of the Birmingham metropolitan area and the two power plants were combined; good agreement was found between the estimated fluxes (2.0 to 5.5×1025 molecules s−1) and the emission inventory (3.7×1025 molecules s−1) for the combined sources. The enhancement of O3 in the urban plume indicates photochemical formation and shows that during the summertime, approximately seven O3 molecules can be formed per NOx molecule added by the urban and power plant emissions. This production efficiency applies both to the isolated urban plume and to the combined urban-power plant plumes and is similar to that derived for rural areas from surface studies. Comparison of the results from several flights indicates the contribution of the regional ozone levels to the O3 concentrations observed within the urban plumes. The aircraft measurements, in combination with surface measurements of ozone, show that the interaction of ozone concentrations entering the urban area, the photochemical formation of ozone during the oxidation of the urban emissions, and the wind speed and direction determine the location and the magnitude of the peak ozone concentrations in the vicinity of this metropolitan area.