This assessment is the fourth and final assessment of technical issues related to ozone and other photochemically generated products formed in the atmosphere requested by the Office of Air Quality Planning and Standards. A survey of information available on the role of NO(sup x) in nonurban O3 formation was prepared in 1979 (Martinez and Singh, 1979). This earlier survey was carried out to examine the hypothesis that NO(sup x) was the limiting precursor species with respect to photochemical formation of O3 over nonurban areas. If this hypothesis could be proved, it would indicate that increases in NO(sup x) concentrations could lead to higher concentration levels of nonurban O3. Because of the fragmentary knowledge of the interactions between NO(sup x) and O3 in nonurban areas, the authors of the survey could not arrive at any definite conclusions. However, this survey also included an interesting analysis of O3 and NO(sup x) measurements obtained from the Sulfate Regional Experiment (SURE) sites in the eastern United States. Additional experimental measurements, laboratory smog chamber results and modeling studies have become available during the last 5 years. These results have been discussed in a review (Altshuller 1984b) prepared on this subject. Based on these recent studies and earlier results a reevaluation of the factors influencing rural O3 formation destruction and the role of the NOx in O3 formation is provided in the present assessment.