Attempts to characterize ozone formation as sensitive to either volatile organic compounds (VOC) or NO_x within a region oversimplify a problem that shows spatial and temporal variation. California's Central Valley has some of the highest ozone levels in the country and a rapidly growing population, and air quality problems in this region can be influenced by interbasin transport of ozone and its precursors. An Eulerian photochemical airshed model is applied to the region for the period 3–6 August 1990. This episode spans a weekend, and the emission inventory incorporates revised motor vehicle emissions with day-specific activity patterns. Compared with the baseline inventory, the revised inventory contains higher VOC and lower NO_x emissions from motor vehicles and different temporal patterns of these emissions. Revised estimates of biogenic emissions are greatly reduced. The baseline emission inventory used in previous modeling efforts appears to contain compensating errors, with high biogenic emissions making up for low motor vehicle emissions of VOC. Results suggest that heavily urbanized areas around and downwind of the San Francisco Bay and Sacramento are VOC-sensitive, while the more rural areas are NO_x-sensitive. Ozone formation in the San Joaquin Valley, where progress in reducing ambient ozone concentrations has been slow, exhibits sensitivity to emissions of both VOC and NO_x, and is influenced by emissions from the San Francisco Bay Area during this modeling episode.