Growth of young cucumber plants was strongly inhibited when Ca was removed from the culture solution. The activities of nitrate reductase, glutamate dehydrogenase and glutamine synthetase were investigated after the removal of Ca. Though the activities of glutamine synthetase and glutamate dehydrogenase were not altered much, nitrate reductase activity, measured by in vitro and in vivo assays, decreased dramatically. The loss of nitrate reductase activity coincided with the level of NO3 in the leaves. When NO3 was supplied to the cucumbers with a NO3 deficiency, the plants induced nitrate reductase together with a distinct accumulation of nitrate. Cucumbers treated for both Ca and NO3 deficiency failed to induce nitrate and to accumulate nitrate on the addition of large amounts of NO3. Leaf sections that had been treated for both Ca and NO3 deficiency could induce nitrate reductase when floated on NO3 solution under the light. The drastic loss of nitrate reductase caused by the removal of Ca was probably due mainly to the deficiency of NO3 as the inducer in leaves.