Nine 1-(3,4-dimethoxyphenethyl)-2-methyl-4-(substituted benzylidene)-5- imidazolones were evaluated for their ability to inhibit the activity of rat brain monoamine oxidase during oxidative deamination of tyramine, 5-hydroxytryptamine (serotonin), and kynuramine. All substituted imidazolones inhibited monoamine oxidase activity. A concentration-dependent inhibition of monoamine oxidase was observed using kynuramine as the substrate and the degree of enzyme inhibition was also evaluated on the basis of their I50 values. Preincubation of imidazolones with the enzyme preparations for varying lengths of time prior to the addition of substrate in no way altered their degree of inhibition and thus exhibited a reversible nature of inhibition. A kinetic study carried out with 1-(3,4-dimethoxy-phenethyl)-2-methyl-4-(4- nitrobenzylidene )-5- i midazolone revealed a competitive nature of inhibition of rat brain monoamine oxidase.