Atmospheric distribution of photochemical oxidants has been a subject of interest and concern not only because of their deleterious effects on human health and vegetation but also because of their crucial role in determining the chemical composition of the atmosphere. Several important issues related to the distribution and production of photochemical species are examined through an analysis of results obtained from applications of a comprehensive three-dimensional regional scale photochemical model over the Northeast United States. The Regional Oxidant Model (ROM) is used to simulate the response of various photochemical species to specific anthropogenic emission strategies involving NOx and hydrocarbon reductions for an episodic period during July 1988. Domain and temporal averages of predicted concentrations are examined for various species. Their relative influence on oxidant chemistry over the modeled domain is investigated. Further, spatial distributions of O3 with respect to those of NOx, NOy, and hydrocarbons over the modeled domain are examined and the variations in O3 levels for different chemical regimes classified by characteristic NOx/reactive organic gases and NOx/NOy ratios are investigated. Temporal trends in domain-averaged concentrations indicate that the model replicates the expected diurnal trends in species concentrations. The relative benefits of reduction in NOx and hydrocarbon emissions on predicted O3 levels are also examined. In general, for this modeled domain, reductions in NOx emissions with or without reductions in hydrocarbon emissions have more impact on reducing predicted O3 levels compared to reductions only in hydrocarbon emissions. ¿ American Geophysical Union 1994