Studies of the transient changes in the maximum catalytic capacities of glucose-metabolizing enzymes in preneoplastic rat liver revealed significant decreases in the activities of only two enzymes, glucokinase and hexosediphosphatase. The activities of hexokinase, glucose-6-phosphate dehydrogenase, and phosphogluconate dehydrogenase increased threefold and there was a significant elevation of liver enzymes in the serum after 45 days of carcinogen treatment.Fluctuations in the in vivo concentrations of glycolytic intermediates reflected an increased glycolytic flux with an activation of the lower segment of the pathway at the pyruvate kinase level. The constancy of the mass action ratios for the glycolytic reactions suggested that there was no impairment of the regulatory status of glycolysis in the preneoplastic period. There was, however, a most notable metabolic impairment in the disruption of the interrelationship between the phosphorylation potential and the cytoplasmic pyridine nucleotide ratio.