DEAE-cellulose chromatography of partially purified preparations of UDP-glucuronate carboxy-lyase from wheat germ results in the separation of two forms of the enzyme. Both are fully active in the absence of added DPN, have indistinguishable molecular weights (210,000), but differ in charge and kinetic properties. Both are cooperatively activated by UDP-glucuronate, however Enzyme 1 is activated at lower concentrations than Enzyme 2. At low substrate concentrations (less than or equal to 5 micron), both enzymes are activated by UDP-glucose, 2 mM concentrations of activator increasing the activity of Enzyme 1 2-fold and of Enzyme 2 2.5-fold. UDP-xylose allosterically inhibits both enzymes. At substrate concentrations equal to the apparent Km values, inhibition of Enzyme 1 is much greater than that of Enzyme 2 (83 and 28% at 0.33 mM inhibitor concentration). The data suggest that synthesis of UDP-xylose is controlled both by substrate activation and product inhibition of UDP-glucuronate carboxy-lyase. The existence of a "more active" and a "less active" species of the enzyme suggests the possibility of two interconvertible forms of the same protein and the involvement of such interconversion in further regulation of UDP-xylose biosynthesis. However it is equally possible that both represent true isoenzymes.