Abstract: Activities of terminal oxidases involved in the respiration of rice seedlings (variety Yinfang) were studied with respect to their adaptation to low oxygen tensions. Four–day old seedlings grown on quartz sand moistened with Espino solution were used as material. The results of these experiments are summarized below. 1. Ascorbic acid oxidase and cytochrorne oxidase, but not polyphenol oxidase are found in the tissue extracts of the seedlings. The activities of these enzymes differ in different organs. Higher cytochrome oxidase activity is found in coleoptiles and leaves while in the root, ascorbic acid oxidase activity is higher than that of cytochrome oxidase. 2. Inhibition experiments with CO, sodium azide and dieca indicate that the metal-containing oxidases (cytochrome and ascorbic acid oxidases) comprise about one half of the total terminal oxidase activity, the remainder is presumably due to that of the flavo-proteins which are insensitive to the actions of these inhibitors. The activities of the 2 metal enzymes, cytochrome and ascorbic acid oxidases are equally intensive, each comprises ¼ of the total terminal oxidase activity of the entire seedling. 3. In seedlings which have been adapted for 24 hours under 21%, 10%, 5% and 1% partial pressures of oxygen, the relative intensities of ascorbic acid oxidase activity is unchanged while the activity of cytochrome oxidase is very much increased, with a corresponding decrease in flavoprotein activity. 4. Under submerged condition, there is an increase in cytochrome oxidase activity in the seedlings and a corresponding decrease in flavo-protein activity much as that which is observed under low oxygen tensions. Withdrawal of the water layer after 24 hours of submergence reversed the situation, so that there is evidence of de-adaptation in these enzyme activities. 5. Low oxygen tension is unfavorable for seedling growth. Under such conditions the dry weight of the seedlings decreases. There is an apparent correlation between this effect of low oxygen on growth and the degree of activity of cytochrome oxidase. Relatively long periods of oxygen depletion are definitely harmful to seedling growth, and it is concluded that growth in the rice seedlings is an oxygen-requiring process.