A field fumigation system was modified from one described elsewhere, and used to expose unenclosed hybrid poplar plants (Populus deltoides x trichocarpa) to low levels of O3 and (or) SO2. A 3x3 factorial design was employed, allowing for analysis of individual effects and potential interactions between the two pollutants. Two experiments were conducted during the summer of 1980. During the experiments, levels of ambient SO2 and O3 were near zero (<0.01 ppm) and low (mean. 0.04 ppm), respectively. Pollutant treatments included exposure to ambient air or to ambient air plus additional O3 and SO2 for 5-6 h/day. Fumigations with O3 and SO2 were made on 17 of 27 (first experiment) and 13 of 20 (second experiment) days, respectively. Exposure to two greater than ambient levels of O3 (mean concentrations, 0.06 or 0.08 ppm) and (or) SO2 (mean concentrations, 0.06 or 0.11 ppm) resulted in reductions in growth, dry matter accumulation, and leaf longevity. No significant interactions between the pollutants were observed and O3 had approximately a three times greater negative effect than SO2 on either a concentration or dose basis. All significant pollutant effects were linear with respect to total dose or average exposure concentration. The negative impact of low doses of O3 suggests a need for concern, since low level O3 pollution is chronic during the growing season in much of eastern North America.