The inhibition D-amino acid oxidase by certain alpha-keto acids products of the reaction with D-amino acids, in particular oc-keto acids that are analogs of the amino acids alanine, valine, leucine, phenylanaline, phenylglycine, tyrosine and tryptophan, is reported All the alpha-keto acids assayed behaved as substrate competitive inhibitors of the enzyme. The relationship between the degree of inhibition and the structure of the inhibitor is discussed.