UDP-glucuronic acid pyrophosphorylase was purified approximately 80-fold from barley seedlings by ammonium sulfate fractionation and column chromatography on DEAEcellulose and Sephadex G-100. The partially purified enzyme can be assayed radiochemically in either the forward or reverse direction and has K, values of 0.33 mM and 0.5 mM for n-glucuronic acid l-phosphate and UDP-D-glucuronic acid, respectively. Its pH optimum lies between pH 8 and 9. The equilibrium constant of the reaction is 2.9 measured in the direction of glucuronic acid l-phosphate formation. Mg*+ best fulfills the requirement for divalent cations. However, this dependency is related to substrate concentration, and, on the basis of kinetic experiments, an Mg*+: UTP ratio of 2: 1 in the forward direction and an Mg*+ to pyrophosphate ratio of about 1: 1 in the reverse direction appear to be required for maximum activity. Excess Mg2+ inhibits the reaction. The total activity of the enzyme increased 15-fold during the lirst 6 days of germination and is present in amounts potentially sufficient to meet the full needs of the young plant for UDP-glucuronic acid in cell wall polysaccharide biosynthesis. By contrast, levels of UDPglucose dehydrogenase were very low. This suggests that the pathway myo-inositol -+ glucuronic acid ---t glucuronic acid l-phosphate + UDP-glucuronic acid may be the predominant route whereby UDP-glucuronic acid is formed in the young barley seedling.