Uric acid seems to act as an electronic acceptor in the dehydrogenation of hypoxanthine catalyzed by chicken liver's xanthinedehydrogenase (XDH). Oxidation was observed in crude homogenates under anaerobic conditions, although dialyzed homogenates or purified hepatic XDH also induce a similar action either in aerobic or anaerobic conditions. The reaction pH optimum is about 6.0. Xanthine appears to be the only inhibited product of the reaction when its concentration is greater than 1 X 10(-4) M. When hypoxanthine and uric acid concentrations exceed 2 X 10(-3) M and 1 X 10(-4) M, respectively, they induce inhibition by substrate. Purine is a fairly good substrate of XDH when uric acid acts as acceptor. Allopurinol inhibits hypoxanthine oxidation by uric acid in the presence of XDH. XDH also catalyzes the dismutation of xanthine to hypoxanthine and uric acid.