For June and July 1975, ozone concentrations throughout the Midwest showed a consistent dependence onwind direction with the highest concentrations associated with winds from the southeast. This systematicpattern suggests that large-scale transport rather than local sources control the general trends of the ozonelevel in the study area.Ozone and meteorological data covering an episode period 29 July-2 August have been analyzed usingweather maps, visibility reports, upper air soundings, trajectories and aircraft measurements to ascertainthe scale of the ozone problem. Results indicate that the highest ozone concentrations occurred within regions of reported haze, and that these regions had definite bounds which could be followed over the country.Moreover, distinct areas of reported obscured sky formed in the vicinity of St. Louis and Pittsburgh whichcould be followed over thousands of kilometers reducing visibilities substantially even in upper Ontario.Trajectories indicate that adverse conditions occurred when stagnant air over the Ohio River valley wastransported northward on the western half of a high pressure system. The haze region was preceded to thenorth and east by extremely high temperatures (>35"C). Vertical ozone profiles obtained by aircraft measurements show that ozone concentrations above the surface inversion in the haze remain quite high over-night, but above the subsidence inversion the ozone was low. It is suggested that the episode was due primarily to large-scale transport of ozone produced from precursors accumulated during the stagnant periodover the eastern Midwest.