IN THE preceding paper Marsh and Goddard (1939) presented results obtained with respiratory enzyme poisons showing that in carrot roots and in young carrot leaves a large fraction of the respiration is catalyzed by an oxidase similar to or identical with the cytochrome oxidase; in mature leaves a shift occurs in the oxidase to a system insensitive to cyanide, azide or carbon monoxide. The present paper is a study of the relation between respiration and in carrot root tissue. It will be shown that inhibition of respiration by lowered oxygen pressure, by cyanide, azide, or carbon monoxide results in Increased fermentation. It has long been known that many higher plants liberate CO2 when placed in pure N2 or other inert gas. Alcohol is also produced in many cases (Kostychev, 1931; Stiles and Leach, 1932). The ratio alcohol/CO2 produced has been investigated for manv plants; considerable variation in this ratio from one plant to another has been found, the values ranging from 0 to 0.99 (Kostychev, 1931). Although there has been much speculation as to probable importance of anaerobic CO2 production in higher plants, very little research has been conducted in the field since the later years of the nineteenth century. It is well known that in yeast (Meyerhof, 1926), animal tissues (Warburg, 1930), and in higher plants (Kostychev, 1931; Blackman, 1928) either no occurs in air or it occurs at a lower rate than under anaerobic conditions. If lactic acid production (in animal tissues) in N2 and in air, and oxygen consumption in air, be converted into terms of sugar decomposition according to the equations (1) C6H1206 -> 2CH3CHOHCOOH and (2) C6H1206 + 602 > 6CO2 + 6H20, it is found in many cases that more sugar is decomposed per unit time in N2 than in air. This phenomenon of an increased rate of sugar decomposition in transferring tissues from air to N2 is known as the Pasteur effect. The literature of the Pasteur effect has been well reviewed by Dixon (1937), Turner (1937), and Burk (1937). Although the transfer of a tissue from nitrogen into air allows oxidative sugar decomposition to occur (equation 2), at the same time there is such a large decrease in the rate of sugar decomposed by glycolysis (equation 1) that the result is an overall decrease in the rate of sugar decomposition. The ratio lactic acid in N2mols lactic acid in air/mols 02 in respiration, is known as the Meyerhof quotient (1926). Values of this quotient lie between 1 and 2 for many animal tissues (Burk, 1937) and for yeast (Meyerhof, 1926). (In the quotient for yeast, the term mols CO2 of fermentation is substituted for mols lactic acid. The expressions