A series of new sympathomimetic amines containing an 8-hydroxycarbostyril moiety was synthesized. These compounds probably exist as resonance hybrids having two acidic hydrogen atoms in locations approximating to those of the hydroxyl groups of catechol-containing adrenergic agents. In an in vitro test, many of these compounds showed potent activity for relaxation of guinea pig tracheal smooth muscle. One of the compounds was 24 000 times more potent than isoproterenol. Their actions on cardiac muscle were also examined in vitro by measuring increase in the beating rate of the right atria of guinea pigs. Several of the compounds appeared to be beta-selective. Some of the compounds seem suitable for use as bronchodilators. The structure-activity relationships of these compounds were discussed in comparison with those of catecholamines.