The sensitivity of regional ozone concentrations to biogenic hydrocarbons in the northeastern United States has been examined using an Eulerian grid photochemical model having a horizontal resolution of 18 km. A 6-day period during which observed ozone concentrations exceeded 200 ppb was simulated. Detailed estimates of biogenic nonmethane hydrocarbon (NMHC) emissions were included in the model simulations. Overall, biogenic emissions were of the same order of magnitude as anthropogenic emissions. Approximately 33% of the biogenic inventory was in the form of isoprene, with the remainder of the NMHC in the form of monoterpenes and unidentified NMHC. Three model sensitivity runs were analyzed in which biogenic emissions, and then anthropogenic hydrocarbon emissions, were selectively removed from the emissions data set. Episode maximum predicted ozone concentrations were compared for each simulation, and a detailed chemical analysis was performed on two trajectories within the modeling domain. The analysis showed that the relative impact of biogenic compared to anthropogenic hydrocarbons on ozone generation varied spatially over the model domain. The biogenic influence on ozone was greatest in western and southern sections of the domain, particularly in the Ohio Valley, while the anthropogenic influence was greatest in the urbanized Northeast Corridor and industrial Great Lakes area.