Measurements from the Regional Air Pollution Study in St. Louis and from synoptic rawinsonde stations in North America were used with annual point and area source emission data in St. Louis to establish some consequences of local and large-scale ozone transport. In rural areas outside of St. Louis, ozone concentrations exceeded the National Ambient Air Quality Standard of 80 ppb (160 micrograms/cu m) and could not be attributed to the emissions of pollutants within the metropolitan area of St. Louis. Typically, these high ozone concentrations occurred when the air flowing into St. Louis had been associated with an anticyclone during the 3 days prior to its arrival. Trajectories indicated that during these 3-day periods the air had remained within the eastern half of the United States where there are numerous high-intensity urban-industrial centers. On other days during the study analyses of the local air flow confirmed that the urban-industrial areas within the immediate vicinity of St. Louis were responsible for high ozone concentration in the city as well as in the rural areas. An analysis of the formation and movement of an 'ozone cloud' through the network of stations revealed the consequences of local ozone generation and transport.