Abstract: Four-to-six-day old seedlings of rice (variety Yin-fang) were used to demonstrate adaptive formation of the enzyme nitrate reductase through substrate induction. These seedling were germinated on quartz sand with M/15 or M/30 phosphate buffer as culture medium. When so grown the seedling do not give nitrate reductase activity. When nitrate is added to the buffer solution or when the germinated seedlings were treated with nitrate solutions for spefified period of time nitrate reductase activity was clearly demonstrable in these seedlings. This is true whether etiolated or green seedlings were used as experimental materals. The optimum conditions for this process of adaptive enzyme formation were found to be: pH 5, 30 ℃, and 6 mM of nitrate (as K and Na salts in equal amounts, in M/15 or M/30 phosphate buffer). Under these conditions nitrate reductase formation in these rice seedlings could be induced in one day for etiolated intact seedlings. In the intact green seedlings, an induction period of only 5 h is sufficient. With isolated roots of these rice seedlings, an induction period of only 3 h is sufficient for the adaptive formation of this enzyme under optimum conditions. When transferred back to nitrate-free media, the adapted seedlings lose their nitrate reductase activity. The time needed for this deadaptation is about 2 d. Ammonium sulfate is ineffective as substrate for inducting nitrate reductase formation in these seedlings. In fact it appears to expert an inhibiting action on its formation in presence of nitrate. Low oxygen tension does not significantly impede nitrate reductase formation in these rice seedling. At an oxygen concentration of 5%, nitrate reductase formation was found to be as intense as when the induction was carried out at 10% oxygen. Even when the induction was carried out at 1% oxygen, significant amounts of the enzyme was formed in these seedlings. These results are explained on the assumption that under such conditions reduction of nitrate by these seedlings furnish sufficient oxygen for their metabolic activity. The fact that under very low oxygen tension (1%) the R. Q. of the seedlings is greater than unity supports our view that under such conditions nitrate may serve as the oxygen donor in the respiratory process of the seedlings. The role of nitrate reductase in this case may be considered to be that of a terminal oxidase. Attention is called to the observation that adaptive formation of nitrate reductase is greatly enhanced in isolated roots to which scutellum tissues are attached. The function of scutellem in seedling metabolism merits further investigation. Rice seedlings were found to take up considerable amounts of nitrate in a nitrogen atmosphere. This appears to be at variance with the "anion respiration" theory of salt up-take. Experiments are being performed to clarify this point. The results of these experiments appear to furnish a convincing demonstration of the adaptive formation of nitrate reductase in rice seedlings (var. Yin-fang). This is perhaps the first clear instance in support of the formation of an adaptive enzyme in higher plants. The significance of this finding in relation to mineral nutrition of the rice plant as well in the general theme of the mechanism of adaptation of plants to their environment are discussed.