Abstract: Three-day old and seven-day old rice seedlings (Oryza sativa L. variety Yinfang) were treated under oxygen-deficient condition (by substituting the air with nitrogen in sealed glass bottles). Localization of cytochrome oxidase and polyphenol oxidase in roots were determined histochemically at fixed intervals after treatment (8, 24, 32, 48, 72 and 96 hours). The results obtained in this investigation may be briefly summarized as follows: 1. Polyphenol oxidase and cytochrome oxidase are detected in the seedlings from the second day of germination. 2. Maximum activity of polyphenol oxidase in the root tip appears on the fourth day of germination but it decreases progressively thereafter. In the vascular bundles of maturation zone, however, the enzyme activity is found to increase continuously with in- crease in age. If oxygen supply is limited or is withdrawn, a sharp decrease of enzyme activity occurs after 8 hours of treatment. It continues to decrease rapidly and almost disappears after about 48 hours. 3. The activity of cytochrome oxidase increases steadily up to the 10th day of germination when it reaches its plateau and is maintained at such a level thereafter. Under the condition of limited oxygen supply, the enzyme activity decreases but it still shows relative increases in activity as the plant grows. It appears likely that adaptive formation of this enzyme occurs during oxygen deficiency. On the basis of the observed results, one may suggest that the adaptation of cytochrome oxidase to limited oxygen supply probably plays an important role in the tolerance of rice to submergence under water. 4. The results show that these two types of terminal oxidases are more sensitive to oxygen on the third day than on the seventh day of germination.