This study examines the relationships between regional ozone (O₃) pollution and emissions of nitrogen oxides (NO _x ) in the eastern United States during summer. Using measurements from rural sites during the summer of 1995, three 4-day time periods are identified during which significant enhancements of surface O₃ occurred on spatial scales ranging from 0.5 to 1.8 million km². Each of these episodes was characterized by relatively stagnant meteorological conditions conducive to the photochemical formation and accumulation of O₃ in the boundary layer. The surface ozone accumulation efficiency (SOAE), a parameter which relates the O₃ accumulation to the NO _x emission density in a given region, is estimated to range from 1 to 2 ppbv O₃ kg⁻¹ N km⁻² in the eastern United States during summer. This result is discussed in the context of regional NO _x -based O₃ control strategies. In addition, the net ozone production efficiency (OPE) is estimated to range from 2 to 3 ppbv O₃ ppbv⁻¹ NO _x in this region.