On the control of carbohydrate utilization in yeast. — The results of a previous investigation showed that in higher plants the stimulating action of 2,4 dinitrophenol (DNP) on oxygen uptake and glycolysis is accompained by a fall of the level of reducing sugars, due to an increase of their respiratory utilization, and thus — according to every evidence — of the rate of hexose phosphate synthesis. In the present work, the occurrence of a similar phenomenon in yeast (where the inhibiting effect of DNP on glucose uptake is not so much marked as in higher plant tissue) was investigated. Here again DNP, at a 10-4M concentration, induced a rapid decrease of the disaccaride trehalose and of glycogen, such as to account for the increased rate of respiration and of fermentation. The ratio between the contributions to CO2 of Carbons 1 and respectively 6 of glucose was not significantly changed by DNP, which suggests that at least part of the DNP induced increase of glycolysis was mediated by the Embden Meyerhof pathway, and thus that a larger amount of fructose diphosphate was formed in the presence of the uncoupler. In other experiments the effects of DNP on the dissimilation of C14 labeled glucose, glycerol and pyruvate to CO2 and ethanol, and on the incorporation of the radioactive isotope into various fractions, 15 minutes after feeding the labeled substrates, was investigated. It was found that:1) Glucose and glycerol uptake is not markedly inhibited by DNP at the concentration employed (10–4M). 2) In the absence of DNP, a considerable portion of the radioactivity fed as glucose or glycerol and taken up by the yeast cells is recovered in the glycogen and trehalose fractions. (35% of the glucose, and 22% of the glycerol taken up). This is also observed for carbons 2 and 3, but not for carbon 1 of pyruvate. This indicates a reversibility of the glycolitic processes comprehended in the region between phospho-enol pyruvate andpolysac-carides; while the pyruvate kinase reaction appears to represent a sharp barrier at the « lower » end of glycolysis. 3) DNP almost completely inhibited the incorporation of C14 from glucose and glycerol into glycogen and trehalose, although it increased the rate of its dissimilation to CO2 and ethanol. The total amount of glucose and glycerol transformed in the various metabolites (and thus — according to every evidence — phosphorylated) was somewhat lowered and proteins synthesis severely depressed. These effects are interpreted as due to the uncoupling action of DNP at the mitochondrial level, and to the consequent general decrease of the ATP and UTP levels required for protein and for polysaccharide synthesis.