The relationships between the monthly averages of daily 1200-1500 h O3 inflow concentrations and the wind flow direction are evaluated using measurements at rural locations outside of St. Louis, Missouri. Although there may be a stratospheric component to these ground level rural O3 concentrations, most of the O3 measured can be associated with photochemical O3 formation in the free troposhere and in the boundary layer. Most of the gradient in rural O3 concentrations from west to east of St. Louis is attributed to higher anthropogenic emissions of O3 precursors to the east rather than to differences in meteorological conditions from west to east.