The inhibition of D-Amino Acid Oxidase by alpha-Keto Acid, product of the reaction with D-Amino Acid is described for the first time. Inhibition of the enzyme by alpha-Keto-gamma-Methiolbutyric Acid (4-methylthio-2-oxobutanoic acid), product of the reaction with D-Methionine, was studied, From the results obtained it is deduced that inhibition is competitive with the substrate, the value of the inhibition constant being 1.85 10(-3) M.