I present the results of an analysis of ozone data from rural locations in the United States. Ozone concentrations above 80 ppb are common in the east in spring and summer, but they are unusual in the west, and ozone shows considerably more day-to-day variability in the east. Variations in ozone levels are highly correlated over distances of several hundred kilometers in the east, indicating that high values are associated with episodes of large spatial scale, >600,000 km2. There were 10 and seven such episodes in 1978 and 1979 respectively, between the months of April and September; they persisted for 3–4 days, on average, with a range of 2–8 days, and were most common in June. Daily maximum ozone values exceeded 90 ppb at over half the sites during these episodes and were often greater than 120 ppb at one or more sites. An analysis of the meteorology for each episode shows that they occurred preferentially in the presence of weak, slow-moving, and persistent high-pressure systems. Two episodes that occurred outside the summer half of the year were associated with unseasonably warm weather; only one episode, in March 1978, appeared to reflect a major stratospheric intrusion. Concentrations of NOx at rural locations in the east are frequently high enough (>1 ppb) to permit significant photochemical formation of ozone. It is clear that rural ozone in the east in spring and summer is severely impacted by anthropogenic emissions of NOx and hydrocarbons, and that ozone episodes occur when the weather is particularly conducive to photochemical formation of ozone. Ozone episodes were present on 23% of days in May-August in the east in 1978–1979. The effect of these pollution episodes on vegetation cannot be assessed with current information on dose-response characteristics, which is based primarily on exposure of crops to a given level of ozone for 7 hours a day. The results presented here may be used to design studies that account for the periodic exposure of vegetation to high values of ozone for several consecutive days.