The respiration and photosynthesis requirement for induction and maintenance of nitrate reductase activity was determined on leaves of Hordeum vulgare L. In this induction, glucose substituted for light in both dark-grown and carbohydrate-depleted green leaves. Oxygen appeared to be required for induction in all cases studied. In light and under N(2), 3-(3,4-dichlorophenyl)-1,1-dimethylurea completely inhibited induction, presumably by inhibiting the production of O(2), Hence, under N(2) the leaves appeared to utilize both the O(2) produced by photosynthesis and the CO(2) produced by respiration. CO(2) fixation can then produce both photosynthate to drive the induction and terminal electron acceptors to allow photosynthetic electron flow. This possibility was further suggested by the observation that CO(2) was an absolute requirement for induction in carbohydrate-depleted barley leaves. Results obtained with respiratory inhibitors also indicated that respiration drove the induction of nitrate reductase.Exogenously supplied glucose also substantially slowed the loss of nitrate reductase that occurred when barley leaves were placed in darkness. It is presumed that glucose allowed the synthetic or activation phase of the induction to proceed more rapidly. Our results support the hypothesis that one of the main effects of light may be to supply photosynthate to support respiration, which then drives the induction process.