High ground-level O-3 is a new threat to agricultural production in Northeast China with the increasing ambient O-3 concentration. Little is known about its impacts on soybean production in this key agricultural region. Accumulated O-3 exposure-response and stomatal O-3 flux-response relationships were developed during two continuous growing seasons to evaluate O-3-induced yield reduction of four typical soybean cultivars in Northeast China. Results showed that critical levels ofAOT40 (accumulated hourly O-3 concentrations over a threshold of 40 nmol.mol(-1)), SUM06 (sum of all hourly average O-3 concentrations over 0.06 mu mol.mol(-1)) and W126 (sum of O-3 concentrations weighted by a sigmoidal function) in relation to 5% reduction in relative seed yield were 4.2, 7.6 and 6.8 mu mol.mol(-1) . h, respectively. The effect of O-3 on plants was influenced by leaf position in canopy. An improved Jarvis stomatal conductance model including leaf (node) position fitted well with field.measurements.The best linear relationship between stomatal 03 flux and relative soybean yield was obtained when phytotoxic ozone dose was integrated over a threshold of 9.6 nmol.m(-2).S-1 (POD9.6) to represent the detoxification capacity of soybean. POD9.6 and the commonly used POD6 in relation to 5% reduction in relative seed yield of soybean were 0.9 mmol.m(-2) and 1.8 mmol . m(-2), respectively. O-3 concentrations above similar to 38 nmol.mol(-1) contributed to POD9.6 and caused seed yield loss in soybean. Current annual yield loss of soybean at ambient O-3 was estimated to range between 23.4% and 30.2%. The O-3 dose-response relationships and corresponding thresholds obtained here will benefit regional O-3 risk assessment on soybean production in Northeast China.