As an exercise in model sensitivity, the Environmental Protection Agency's regional oxidant model (ROM) was run for several simulations to study the impact of across-the-board reductions in anthropogenic volatile organic compound (VOC) and NOx emissions on the photochemical environment of the eastern United States. The ROM, which simulates most of the physical and chemical processes responsible for the formation of O3 on regional scales, was used to simulate a widespread high O3 episode in the eastern United States. Separate simulations were performed over the period July 2--10, 1988, for each of the sensitivity runs. An operational evaluation and several model diagnostics were performed on the base case simulation. The sensitivity runs reduced anthropogenic NOx and VOC emissions separately and in combination in increments of 25% of full scale emissions. Biogenic emissions were held constant across all sensitivity tests. Seventeen of the potential 25 nodes of the NOx, VOC emissions reduction matrix were simulated. In the analysis the modeled domain (the eastern half of the United States) is subdivided into several chemically coherent regions. Several chemical species, including afternoon average concentrations of O3, PAN, HNO3, OH, and HO2+RO2, are examined to gain an understanding of how the reduction in emissions affected the overall chemical system for each sensitivity test. Results are presented as a series of statistical and graphical displays illustrating the response of the system within the selected subdomains to the emission reductions. ¿ American Geophysical Union 1995