Abstract. Seedlings of Zea mays L. were grown in the dark at 27°C. Four‐day‐old seedlings were then exposed for 3 days to solutions equilibrated with gas mixtures to give O2 concentrations between 0.02 and 0.25 mol m−3. Root growth was impaired just as severely at 0.06 as 0.02 mol O2 m−3 while growth at 0.16 mol O2 m−3 was about the same as in solutions in equilibrium with air (0.25 mol O2 m−3). Growth of young seedlings at low O2 concentrations was inhibited to the same extent in nutrient solution and 0.5 ml m−3 CaCl2, showing that the adverse effect of O2 deficits on growth was not due to less uptake of inorganic nutrients. Furthermore, at low O2 concentrations neither exposure of the shoots to a relative humidity of 100% (26.0 g H2O m−3) nor excision of the entire shoot enhanced root growth relative to that in plants with shoots at a relative humidity of 50% (13.0 g H2O m−3). Therefore, for these seedlings growing in the dark, impairment of root growth at low O2 concentrations was not a consequence of water deficits in the shoot or of other shoot‐root interactions. Total soluble sugars and amino acid concentrations were generally greater at low (0.02–0.06 mol O2m−3) than at high O2 concentrations (0.16–0.25 mol O2 m −3). This applied specifically to the root apices (0–2 mm) and expanding (2–15 mm) tissue except in some experiments where sugar concentrations in expanding tissue were slightly greater at high than at low O2 concentrations. Critical O2 pressures for respiration of excised root segments were approximately 0.117 and 0.065 mol O2 m−3 in the expanding and expanded zones of the roots, respectively. In contrast, the critical O2 pressure exceeded 0.20 mol O2 m−3 in the apex, suggesting that O2 supply for metabolic processes is most likely to be sub‐optimal in this zone. Our results show clearly that the adverse effects of low O2 concentrations are unlikely to be a consequence of substrate shortage for either respiration or synthesis of macromolecules; low rates of ATP regeneration in growing root tissues are the logical cause for impaired growth in young seedlings while they are being sustained by seed reserves.