The relationship between CO2 fixation and growth is a matter of considerable interest. Molliard (6) was probably the first to report that CO2 is beneficial for the elongation of pea stems and lupin hypocotyls in darkness as well as in light. Very little attention was paid to this observation until Yamaki (13) demonstrated that a 100 to 200 % increase in elongation of Avena coleoptiles was effected when as little as 0.5 % CO2 was present in the experimental atmosphere. Similar promotions of growth in coleoptiles and internodes of Avena have also been obtained by Nitsch and Nitsch (8) in certain experiments. More recently Mer (4, 5) has shown that in intact seedlings a concentration of 5 % CO2 promoted mesocotyl growth but inhibited the extension growth of coleoptiles. This inhibition is overcome by nitrates supplied to the seedlings. An investigation was therefore undertaken to study the products of C1402 fixation in relation to auxininduced growth and inhibition in coleoptile sections as also in intact plants. This paper is concerned with a study of the changes occurring in the detached coleoptile. During the progress of this investigation a brief account of Yamaki's recent work was made available (14). His conditions were different from those employed by us but there is a large measure of agreement in the conclusions drawn, namely, that IAA stimulates CO. fixation and has a marked influence on the metabolism of the fixation products.