The reaction catalyzed by calf liver uridine di-phosphate glucose synthase(pyrophosphorylase)(EC 2.7. 7.9; UTP+ glucose 1-phosphate= UDP-glucose+ PPj) is an example of an enzymic reaction in which a nucleoside triphosphate other than ATP is the immediate source of metabolic energy. Kinetic properties of the enzyme, acting in the direction of UDP-glucose formation, were investi-gated in vitro. The reaction was inhibited by UDP-glucose (0.072), Pj (11), UDP (1.6), UDP-xylose (0.87), UDP-glu-curonate (1.3), and UDP-galacturonate (0.95). The num-bers in parentheses indicate the concentration (mM) re-quired for half-maximal inhibition under the conditions