HOKU-81 is one of the metabolites of tulobuterol. Effects of HOKU-81 on isolated trachea and atria of guinea pigs were compared with those of various bronchodilators. HOKU-81 appears to be a potent and selective beta 2-stimulant with a slight inotropic action. HOKU-81 was approximately 8 times more potent than tulobuterol, approximately twice as potent as salbutamol, and approximately as potent as isoprenaline in relaxing effect on the isolated tracheal smooth muscle preparation of guinea pigs. This effect of HOKU-81 seems to be due to direct action on the adrenergic beta-receptor.