Abstract Surface ozone concentrations recorded by the Regional Air Monitoring System (St. Louis) during 1975–1976 are presented in time series form. The plotted concentrations are the daily maximum 1-h average and the daily inflow, that representative of air flowing into the metropolitan region during a 4-h midday period. In general, the inflow varies from 30 ppb in the winter to 70 ppb in the summer, but under strong high pressure systems may approach or exceed 120 ppb. Evidence of local ozone production is apparent, with the highest recorded maximum in the two years being 255 ppb. Scatter diagrams are used to display the dependencies of ozone concentrations on temperature, solar radiation and wind speed. A strong dependence on temperature is particularly notable.