This investigation presents metabolic evidence to show that in 4- to 5-day-old roots of maize (Zea mays hybrid GH 5010) exposed to low external O2 concentrations, the stele receives inadequate O2 for oxidative phosphorylation, while the cortex continues to respire even when the external solution is at zero O2 and the roots rely solely on aerenchyma for O2 transport. Oxygen uptake rates (micromoles per cubic centimeter per hour) declined at higher external O2 concentrations in excised segments from whole roots than from the isolated cortex; critical O2 pressures for respiration were greater than 0.26 moles per cubic meter O2 (aerated solution) for the whole root and only 0.075 moles per cubic meter O2 for the cortex. For plants with their shoots excised and the cut stem in air, ethanol concentrations (moles per cubic meter) in roots exposed to 0.06 moles per cubic meter O2 were 3.3 times higher in the stete than in the cortex, whereas this ethanol gradient across the root was not evident in roots exposed to 0 moles per cubic meter O2. Alanine concentrations (moles per cubic meter) in the stele of roots exposed to 0.13 and 0.09 moles per cubic meter O2 increased by 26 and 44%, respectively, above the levels found for aerated roots, whereas alanine in the cortex was unchanged; the increase in stelar alanine concentration was not accompanied by changes in the concentration of free amino acids other than alanine. For plants with their shoots intact, alcohol dehydrogenase and pyruvate decarboxylase activities (micromoles per gram protein per minute) in roots exposed to 0.13 moles per cubic meter O2 increased in the stele by 40 to 50% over the activity in aerated roots, whereas there was no appreciable increase in alcohol dehydrogenase and pyruvate decarboxylase activity in the cortex of these roots. More convincingly, for roots receiving O2 solely from the shoots via the aerenchyma, pyruvate decarboxylase in the cortex was in an "inactive" state, whereas pyruvate decarboxylase in the stele was in an "active" state. These results suggest that for roots in O2-free solutions, the aerenchyma provides adequate O2 for respiration in the cortex but not in the stele, and this was supported by a change in pyruvate decarboxylase in the cortex to an active state when the O2 supply to the roots via the aerenchyma was blocked.