The first generation U.S. EPA Regional Oxidant Model (ROM1) has been tested and evaluated for O3 predictions on a two-day test case episode (3-4 August 1979) in the Northeast Regional Oxidant Study (NEROS) area. The two-day episode was characterized by relatively high O3 concentrations in the southern Great Lakes area where clear skies persisted. Intermittent periods of relatively high O3 levels existed near some of the eastern seaboard cities where partly cloudy skies alternated with clouds and showers in the vicinity of a stationary frontal zone. Model evaluation results for O3 are presented in two stages. First an analysis of model predictions at all hours of the simulation for all surface monitoring site locations is performed. Results show an average underprediction of only a few percent. In the second stage, an analysis of the ability of the ROM1 to predict daily maximum concentrations, two aspects are considered in the evaluation analysis. First, the maximum O3 concentrations at monitoring station locations are paired with the corresponding model predictions. Second, the model predictions are analyzed for individual urban emissions source-area plumes.