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Nitrate and dark stress were used to induce decline in nitrogenase activity of root nodules of soybeans (Glycine max (L.) Merr. cv. Bragg). Response to this treatment and to various oxygen concentrations was assessed by short-term acetylene reduction assays. Decreases in oxygen partial pressure reduced nitrogenase activity substantially in unstressed soybean nodules and even further in nodules of nitrate- or dark-stressed plants. Increasing oxygen concentrations from 21 to 60% severely restricted nitrogenase activity in control plants, but stimulated activity in soybeans exposed to 10 mol m-3 nitrate or darkness for 2 days. Less stressed plants (treated with 7.5 mol m-3 nitrate) were stimulated less by high oxygen tension. Soybeans deprived of light for 4 days were symbiotically inactive and did not respond to raised O2 concentrations. Bacteroids isolated from these plants had lost their autonomous activity but recovered in the presence of succinate, indicating substrate deprivation. The data presented suggest that suboptimal oxygen concentration near the bacteroids is an important factor limiting nitrogenase activity in nitrate-or dark-stressed soybeans.