A linear gradient field exposure system was modified from one originally described by Shinn et al. (1977) and used to expose field grown soybeans (Glycine max cv Hark) to a concentration gradient of a mixture of two gaseous pollutants: SO2 and 03. Since this technique does not use enclosures, study plants experienced near ambient fluctuations in environmental conditions, including wind, and hence were exposed to widely fluctuating pollutant concentrations. Plants in the gradient system were exposed to both pollutants for 57 h on 12 days during the pod-filling period (31 August-17 September). Mean concentrations during the 57 h of exposure at the 'high' end of the gradient were 0.16 and 0.06 μl l-1 (PPM) SO2 and O3, respectively, with 10 h at greater than 0.25 and 0.10 μl-1 SO2 and O3, respectively. Total doses for these plants were estimated to be 9.0 and 3.5 μl-1 · h SO2 and O3, respectively. Comparison with plants exposed to ambient air indicated that exposure to SO2 and O3 reduced total yield per plant and dry mass per bean by as much as 36 and 15 %, respectively. Since concurrent exposure to a much higher dosage of SO2 alone (20.2 μl l-1 · h) was observed in a separate experiment to have no significant effect on yield, 03, although present at moderately low levels, was probably responsible (alone or synergistically with SO2) for the greatest reduction in seed size and yield.