Responses of established rice plants to anaerobiosis were examined by flushing nitrogen through culture solutions. Although shoot growth was not affected, anaerobiosis reduced root dry weight and caused shortening and increased branching of roots. Oxygen uptake, CO2 evolution and respiratory quotient of excised apical root tissue were lower for plants adapted to anaerobiosis than for non-adapted plants. Anaerobic treatment of roots inhibited chloride, dihydrogenphosphate and potassium uptake by both adapted and non-adapted plants, the inhibitions usually being more pronounced in non-adapted plants. Oxygen supply to the roots was prevented by creating an atmosphere of nitrogen around the shoots; this induced inhibitions of ion uptake additional to the inhibitions observed with anaerobic treatment of roots alone. Ion uptake of adapted plants under anaerobiosis was similar to that of non-adapted plants under continuous aeration. This was shown to be due to a higher capacity for ion uptake by adapted plants, since when adapted plants were returned to aerated solutions ion uptake was higher than for plants with continuous aeration. Thus inhibition of ion uptake, due to both smaller roots and anaerobiosis, was counterbalanced by this increased capacity for ion uptake. It is concluded that oxygen movement to roots from aerial parts does occur in rice, but is insufficient to sustain the full potential of at least some metabolic processes.