Two contradictory models have been proposed for the binding mode of the substrate xanthine to and its activation mechanism by xanthine oxidoreductase. In an effort to distinguish between the two models, we determined the crystal structures of the urate complexes of the demolybdo-form of the D428A mutant of rat xanthine oxidoreductase at 1.7 angstrom and of the reduced bovine milk enzyme at 2.1 angstrom, the latter representing a reaction intermediate. The results clearly indicate the catalytically relevant binding mode of the substrate xanthine.