1. Xanthine oxidoreductase was isolated from toad Bufo viridis (a mainly ureotelic amphibian species) and partially purified. The enzyme occurred as a stable xanthine: NAD+ oxidoreductase (EC 1.1.1.204), unconvertible to the oxidase form. 2. Some properties of the enzyme resembled those of xanthine oxidoreductase from an ammonotelic fish, Cyprinus carpio, and the ureotelic rat, but in other aspects it was similar to this enzyme from an uricotelic snake, Natrix natrix. 3. Inhibition of the toad enzyme by NADH at high non-physiological concentrations rules out a modulation of its oxypurine-hydroxylating activity by in vivo changes in the NADH/NAD+ ratio. Therefore, toad xanthine oxidoreductase plays no regulatory role in the purine nucleotide metabolism.