The U.S. Environmental Protection Agency Regional Oxidant Model, ROM2.2, was applied to a 210 July 1988 episode to test the regional episodic ozone response to different combinations of the across-the-board nitrogen oxides (NOx) and volatile organic compound (VOC) reductions in the eastern half of the United States. Geographical variations on the regional ozone responses to the across-the-board NOx and VOC reductions are investigated. Biogenic influences are also discussed. Subject to the limitation imposed by current input and model accuracy, the ROM results suggest that reduction of NOx emissions is a key factor in reducing regional ozone. The primary benefit of VOC reductions appears to be in reducing ozone peak values near NOx-source-intensive areas, such as large urban centers and/or large NOx point sources. In these NOx-source-intensive areas, a strategy featuring a combination of both NOx and VOC reductions appears to be most effective in reducing the regional ozone. Biogenic VOC emissions play an important role because of their high reactivities and large spatial coverage.