Pharmacological studies on the tetrahydroisoquinoline derivatives isolated from various plants including Phellodendron amurense Rupr. showed short-lasting blocking effects on the ganglionic and skeletal muscle synapses (1-5). In addition, the compounds potentiated the effects of catecholamines. Xylopinine, a semi-synthetic product of phellodendron, showed long-lasting sedative and hypotensive effects in experimental animals (6, 7). The latter effect was ascribed to the block of the adrenergic alpha-receptors. Recently, Iwasawa and Kiyomoto (8) have presented an extensive study on the relationship between the chemical structures of a series of tetrahydroisoquinolines and their pharmacological effects. Among the derivatives, 1-(3, 4, 5-trimethoxybenzyl)-6, 7-dihydroxy-1, 2, 3, 4-tetrahydro-isoquinoline proved to be a potent hypotensive and bronchodilating agent. The inhibition by pronethalol of its stimulating effect on cardiac rhythm indicates the stimulation of the adrenergic beta-receptors in the heart. Following an introduction of pyrogallol as a pharmacological or biochemical tool for the inhibition of catechol-O-methyltransferase (COMT) activity (9), Hattori et al. (10) have shown that trihydoxybenzene moiety other than pyrogallol inhibits also the enzyme activity. Cahen (11, 12) and Inoue (13, 14) have demonstrated the enhancing effect of trihydroxybenzenes on the responses of the adrenergically innervated organs to stimulation of the adrenergic nerve and to administration of catecholamines. Although the distribution of COMT in the body is not fully understand, those organs such as intestine, bile duct, ureter and sphincter muscle of the urinary bladder, which respond to catecholamine with motor inhibition, are supposed to contain a definite activity of the enzyme. The adrenergic stimulating nature of tetrahydroisoquinoline in the beta-receptor origin promted the present experiments, in which the effect on the activity of the rat liver COMT were studied in vitro. Additional studies were made on the activity of rat liver monoamine oxidase (MAO), because the enyme is also responsible for inactivation of catecholamines and the inhibition might result in a potentiation of the action of the amines.