Central infected zone tissue of soybean (Glycine max L. Merr.) nodules was fractionated into separate subcellular compartmentsusing density gradient centrifugation in nonaqueous solvents to better understand how exposure to Ar:O2 (80:20%, v/v)atmosphere affects C and N metabolism, and to explore a potential role for adenylates in regulating O2 diffusion. Whennodules were switched from air to Ar:O2, adenylate energy charge (AEC) in the plant cytosol rose from 0.63 6 0.02 to 0.73 60.02 within 7 min and to 0.80 6 0.01 by 60 min. In contrast, AEC of the mitochondrial compartment of this central zone tissueremained high (0.80 6 0.02 to 0.81 6 0.02) following Ar treatment while that of the bacteroid compartment was unchanged, at0.73 6 0.02, after 7 min, but declined to 0.57 6 0.03 after 60 min. These results were consistent with a simulation model thatpredicted Ar:O2 exposure would first reduce ATP demand for ammonia assimilation and rapidly increase cytosolic AEC,before the Ar:O2-induced decline mediated by a decrease in nodule O2 permeability reduces bacteroid AEC. The possibilitythat adenylates play a key, integrating role in regulating nodule permeability to oxygen diffusion is discussed.