The sensitivity of regional ozone concentrations to biogenic hydrocarbons in the Northeastern U.S. is examined. A six day period from July 1980 is simulated with an Eulerian grid photochemical model having a horizontal resolution of 19 km, using detailed estimates of biogenic NMHC emissions. Overall, biogenic emissions are on the same order of magnitude as anthropogenic emissions. Approximately 33% of the biogenics inventory is in the form of isoprene, with the remainder of the NMHC in the form of monoterpenes and unidentified NMHC. An analysis of three model sensitivity runs in which biogenic emissions, and then anthropogenic hydrocarbon emissions, were selectively removed from the emissions data set, 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, and anthropogenic influence was greatest in the urbanized Northeast Corridor and industrial Great Lakes area.