We synthesized several prodrugs of glycine and γ-aminobutyric acid. In order to establish a screening system from the prodrugs of selective activity to MAO-A or MAO-B, we examined purification conditions such as solubilization with Triton X-100, precipitation with ammonium sulfate, gel filtration and anion exchange chromatography. MAO-B was purified from various tissues such as guinea pig brain, kidney and spleen. MAO-A from human placenta without MAO-B was unstable in above purifications and used as crude. At each purification step, we checked sensitivity of the enzyme to specific inhibitors by developing a convenient fluorescence assay, in which hydrogen peroxide produced by the enzyme was reacted with p-hydroxyphenylpropionic acid. A fluorescence microplate reader measured a fluorescence of the fluorescent product from p-hydroxyphenylpropionic acid with horseradish peroxidase. In comparison with milacemide, N, N-bis(carbamoylmethyl)- N-pentylamine was the best and exclusive substrate for MAO-B. 2- N-(phenylethylamino)-acetoamide was the good substrate for MAO-A and MAO-B same as milacemide. 4- N-( n-Pentylamino)-butyric acid and 4-( N-phenylethylamino)-butyric acid were the moderate substrates for both enzymes, which should release γ-aminobutyric acid. These drugs will be new leading compounds.