meso-Diaminosuccinic acid, a natural antagonist of aspartic acid, is a good substrate for beef kidney D-aspartate oxidase. The oxygen consumption and the ammonia production are in good agreement with the stoichiometry of a typical oxidative deamination. The deamination involves only one of the two amino groups of diaminosuccinate, and is followed by decarboxylation of the first reaction product, 2-amino-3-oxosuccinic acid. By condensation of the compounds thus originating from this latter, pyrazine 2,5-dicarboxylic acid and pyrazine 2,6-dicarboxylic acid are formed. DL-Diaminosuccinic acid is not oxidized by D-aspartate oxidase, nor does it inhibit the enzyme activity.