Abstract: The tyramine colorimetric method was employed for evaluating monoamine oxidase inhibitor (MAOI). Forty-nine compounds (including hydrazines, derivatives of aminoacid and guanidine, and alkaloids of Amaryllidaceae have been examined in vitro and in vivo. It was found that the most promising compound in respect of inhibitory effect on MAO with low toxicity is 4-thiaheptane-1, 7-di-oyl-bis-isopropyl-hydrazine (Tapha, or CH-6). A single administration of 25 mg/kg of Tapha had an inhibitory activity as powerful as that of phenelzine. Its acute LD₅₀ in mice was 5170mg/kg, whilst that of phenelzine was 87mg/kg. Thus, Tapha has a much wider therapeutic range than phenelzine. It is suggested that Tapha has a great safety margin and is worth while for clinical trials.