Abstract: Nitrate reductase activity of several poplar clones were studied under the controlled condition both in greenhouse and field in 1986-1987. Nitrate reductase activity was minimum in the young leaves, while maximum in the mature but no senescence leaves. Studied on two poplar clones indicated:optimum pH value of NR reaction solution was 7.5 or 8.0. There are nitrate reductase in the leaves, leaf stalks and barks. The highest NRA appeared in leaves of upper shoot up to 1.185μmol NO2-g·h, the lowest in the basement:0.545μmol NO2-/g·h and with the moderation appeared in the middle (1.037μmol NO2-/g·h) In contrast, the minimum value of NRA appeared in the upper leaf stalks and barks, smaller in the middle and the maximum appeared in the lowest. The NRA of leaves contained 67.6-69.3% of total NRA of aerial parts of plants, while barks and leaf stalks contained only 26.6-28.5%, and 4.1-5.7% respectivily. The activity of nitrate reductase changes with the growth rhythm in the seasons.