Ambient air quality standards and control strategies are implemented to protect humans and vegetation from adverse effects. We used a process-based tree-growth model (TREGRO) to show that over the past 37 years, changes in O(3) exposure, with accompanying variation in climate, are reflected in changes in the growth of Pinus ponderosa Dougl. ex Laws. in the San Bernardino Mountains near Los Angeles, California, USA. Despite variation in temperature and precipitation over the study period (1963-1999), O(3) exposure consistently reduced simulated tree growth. Simulated growth reductions increased concurrent with increasing O(3) exposure. The maximum growth reduction occurred in 1979. As O(3) exposures decreased during the 1980s and 1990s, effects on growth also decreased. This implies that emission control strategies taken to reduce exposures to attain O(3) standards benefited P. ponderosa growth in the San Bernardino Mountains. This modeling approach provides a powerful tool for solving the difficult problem of evaluating regulatory effectiveness by simulating plant response using long-term climate and air pollution exposure records for a given region.