We examine the sensitivity of ozone concentrations in rural areas of the United States to emissions of NOx and hydrocarbons using a regional photochemical model. Ozone production in rural areas appears to be limited by the availability of NOx. Rural ozone is strongly dependent on emission rates for NOx but is almost independent of hydrocarbons. This relationship is quite different from that in urban air, where ozone levels depend on both NOx and hydrocarbons. The predicted relationship between ozone and nitrogen oxides appears to be consistent with observations in rural air. For the low NOx regime (< 2 ppb) in rural areas, increases in NOx lead to increases in OH and to corresponding increases in the oxidation rate of hydrocarbons and in levels of ozone. Ozone concentrations in urban plumes appear to be related to regional scale production in addition to production within the plume.