Nitrate anions are shown to be very effective inhibitors of glucose oxidase at acidic pH, much more so than halide anions. The inhibition is fully reversible. This effect could be used to extend the concentration range of glucose electrodes up to 0.556 M (100 g/l) at pH 4 in 0.5 M KNO3 and glucose could be assayed in undiluted fruit juices. The disadvantages of this method are: prolonged time of one measurement; and the loss of accuracy owing to very low response of the electrode.