Nitrate reductase (NR, EC 1.6.6.1) activity in wheat( Triticum aestivum L.) leaves changed rapidly and reversibly during light/dark transitions. Darkness decreased and light increased the activity of nitrate reductase. NR extracted from leaves in dark showed more sensitive to inhibition by Mg 2+ than that from lighted leaves. The NR from darkened leaves and lighted leaves showed marked different in the time course of nitrate reduction. These results indicated that the darkness treatment could change the modulation characteristics of nitrate reductase.