Leaf nitrate reductase (EC 188.8.131.52, NADH: nitrate oxidoreductase) (NR) activity decreased on exposure to low irradiance and the decrease was dependent upon duration of exposure. The observed decrease was associated with a decrease in the rate of xylem exudation and nitrate concentration resulting in lesser supply of nitrate to the tops. Continuous nitrate supply to excised leaves in shade or even in the dark resulted in a high NR activity, suggesting that lack of reducing power was not the main cause for lower NR activity. Further, when excised leaves were deprived of nitrate, even in light, NR activity decreased with time. In leaves with their petioles dipped in water, the NR activity decreased and on retransfer to KNO3 within 105 min the activity increased, suggesting that continuous nitrate supply was essential for maintaining high NR activity.