Chicken liver xanthine dehydrogenase can be partially reduced by either xanthine or NADH. Reduction to approximately the 2-electron-reduced level occurs with NADH, and reduction beyond the 2-electron level occurs with xanthine. In both cases, the reaction is triphasic. The first and third phases are dependent on reductant concentration, whereas the second phase is not. Oxidation of fully (6-electron) reduced xanthine dehydrogenase by either urate or NAD is monophasic and dependent on the oxidant concentration. Oxidation stops at about the same level of reduction that was reached by the corresponding reductant. The position of this end point is sensitive to the potential of the reactants but is relatively insensitive to excess concentrations of oxidant or reductant. NADH binding to 2-electron-reduced enzyme is implicated in fixing the end point position in those reactions involving pyridine nucleotides, whereas urate binding is involved in fixing the end point of those reactions involving xanthine and urate.