The pharmacology and hemodynamics of tazolol (1-iso-propylamino-3-(2-thiazoloxy)-2-propranolol HC1), a selective myocardial beta-stimulant, were studied in pentobarbital anesthetized dogs. Tazolol, i.v., increased myocardial contractile force and heart rate, but induced only minimal changes in arterial pressure. The cardiac effects of tazolol were blocked by pretreatment with the beta-blockers propranolol or practolol and inhibited in animals made tachyphylactic to amphetamine. They were not altered by pretreatment with hexamehtonium, atropine or reserpine. Tazolol increased left circumflex coronary artery flow to a far greater extent than a dose level of isoproterenol which produced a greater increase in cardiac output. Tazolol also increased superior mesenteric artery flow, whereas isoproterenol decreased it. Renal artery flow was not altered. Tazolol was also found to be orally active and to possess some mild general beta-blocking activity.